UNIVERSITY OF PANGASINAN
Department of Electronics Engineering
ELECTRONICS ENGINEERING 2
Prepared by: Engr. Cezar N. Velasco Jr.
Amplifier Operation
 An amplifier is an electronic circuit which is capable of increasing the signal
magnitude or amplitude without appreciably altering the signal waveform.
 The biasing of a transistor is pure dc operation. The purpose of biasing is to
establish a Q-point about which variations in current and voltage can occur in
response to an ac input signal.
 To make the transistor work as an amplifier, it is to be biased to operate in the
active region, i.e. base-emitter junction is to be forward biased, while basecollector junction to be reversed biased.
 Output of amplifier is linearly proportional to the input.
Four Possible combination of input-output relationship:
1. Voltage-controlled Voltage source – the output is a voltage that is a function of
input voltage.
2. Current-controlled Current source – the output is a current source that is a
function of input current.
3. Voltage-controlled Current source – the output is a current source that is a
function of input voltage.
4. Current-controlled Voltage source – the output is a voltage source that is a
function of input current.
General Category of Amplifiers
1. Function
2. Frequency response
Classification by Function
1. Voltage Amplifier – is an amplifier in which the output signal voltage is larger
than the input signal voltage. The amplifier amplifies the voltage of the input
signal.
2. Power Amplifier – is an amplifier in which the output signal power is greater
than the input signal. This amplifier amplifies the power of the input signal.
Classification by Frequency response
1. Audio amplifier – is designed to amplify frequencies between 15 Hz and 20 kHz.
These frequencies are within the audio frequency spectrum.
2. RF amplifier – is amplifier designed to amplify frequencies between 10 kHz and
100,000 MHz. No single amplifier can amplify the entire RF range but any
amplifier whose frequency band is included in the rf range and is considered RF
amplifier.
3. Video amplifier – is an amplifier designed to amplify a band of frequencies from
10 Hz to 6 MHz. This amplifier is also called wide-band amplifier because of such
a wide band of frequencies.
General classification of amplifiers based on controlling input parameter and output
controlled sources
1. Voltage amplifier – voltage-controlled voltage source. (Op-amps)
2. Current amplifier – current-controlled current source. (BJT)
3. Transconductance amplifier – voltage-controlled current source. (FET and
vacuum tubes)
4. Transimpedance amplifier – current-controlled voltage source. (current-tovoltage converter)
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UNIVERSITY OF PANGASINAN
Department of Electronics Engineering
ELECTRONICS ENGINEERING 2
Prepared by: Engr. Cezar N. Velasco Jr.
Bipolar Junction Transistor Amplifier Configuration
To analyze the ac operation of a transistor amplifier, we need an ac equivalent
circuit for a transistor. We need a model for the transistor that stimulates how it
behaves when an ac signal is present.
Model – is a combination of circuit elements, properly chosen, that best approximates
the actual behavior of a semiconductor device under specific operating conditions.
re = 26mV⁄ID
 An ac emitter resistance. It is very important because it determines the voltage
gain. The smaller the emitter resistance, the higher the voltage gain.
Bias Confuguration
Zi
Zo
Av
Ai
Phase
Shift
Medium
Medium
High
High
1800
Medium
Medium
High
High
1800
High
Medium
Low
High
1800
High
Low
Low
High
Inphase
Low
Medium
High
Low
Inphase
Medium
Medium
High
High
1800
High
Medium
High
High
1800
CE Fixed-bias
Voltage Divider Bias
CE Emitter Unbypassed
Emitter Follower
Common Base
Collector Feedback
Collector DC Feedback
Input and Output Characteristic Impedance of Transistor Amplifier Configuration
(Approximate, disregard circuit components)
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UNIVERSITY OF PANGASINAN
Department of Electronics Engineering
ELECTRONICS ENGINEERING 2
Prepared by: Engr. Cezar N. Velasco Jr.
1. Common Emitter
 Medium input and medium output impedance
2. Common Collector (emitter follower)
 High input and low output impedance
3. Common Base
 Low input and High output impedance
Classes of Amplifiers (Power Amplifiers)
 The CLASS OF OPERATION of a transistor amplifier is determined by the
percent of time that current flows through the transistor in relation to the input
signal.
 Amplifiers are categorized depending upon the amount of signal that flows at
the output with a given full cycle input signal.
1. Class A
 The amount of the output signal flow varies for a full 360o of the cycle.
 The DC bias level (q-point) is usually set at one half of the supply voltage,
so that the output signal can have a maximum output swing
 The maximum efficiency is 25%, 50% can be obtained using transformer
coupling.
 Class A operation is the least efficient class of operation, but provides the
best fidelity.
 Distortion is low.
 Operate in the active region (linear portion).
2. Class AB
 The output signal flows for more than 180o but less than 360o.
 The Q-point is set between class A and class B level.
 It is at the active region but near cut-off.
 Efficiency is less than 78.5%
 Distortion is moderate
 The complementary class AB amplifier is the most commonly used output
stage. Its circuit operation is similar to that of a class B amplifier, except
that the transistors have a slightly positive bias so that a Q-point DC
current flows even when the input voltage is zero.
 Class AB exhibits an offset output voltage at zero input voltage. However,
feedback can be applied to reduce the offset voltage.
 A quasi-complementary amplifier uses a composite pnp transistor, which
can deliver higher output power than a normal pnp device.
3. Class B
 The amount of the output signal flow is 180o
 The Q-point is set at the cut-off region, but for practical amplifiers, the Qpoint is set a little above cut-off to eliminate crossover distortion.
 The maximum efficiency is 78.5%
 A full 360o output can be obtained by using two class B amplifiers, one
amplifiers produce positive portion (0-180o) and the other amplifies the
remaining negative portion (180o-360o). And this is called push-pull
amplifiers.
 Distortion is high.
 Most commonly used audio power amplifier.
 Because of B.E. voltage drops, a push-pull amplifier exhibits a dead zone
in the transfer characteristic, which increases distortion of the output
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UNIVERSITY OF PANGASINAN
Department of Electronics Engineering
ELECTRONICS ENGINEERING 2
Prepared by: Engr. Cezar N. Velasco Jr.
voltage. The crossover distortion and nonlinearities can be reduces
practically to zero by applying feedback.
4. Class C
 The output signal flows for less than 1800
 The Q-point is set below cut-off region.
 For a common emitter amplifier, the base emitter junction is initially
reverse bias.
 Efficiency is not important factor, for it is not designed to deliver large
amount of power, but it is the most efficient class of amplifiers.
 Maximum efficiency is more than 90%.
 Very high distortion.
5. Class D
 Amplifier used in applications that are not amplitude sensitive, such as in
pulse circuit in FM system.
 The efficiency of class D is comparable to class C.
 Uses transistors as switches where the only modes are switch on and
switch off. It is used in different switching circuits.
 <180o
 <50%
6. Class S
 Can be used to amplify both AM and FM system.
7. Class E
 Similar to Class D.
 <180o
 <50%
Analog Design
 Class A, B, AB, and C. Based on the shape of collector current waveform
for a BJT or the shape of the drain current for a MOSFET in response to a
sinusoidal input signal.
 In analog amplifier, the input signal to the amplifying devices causes a
proportional output current to flow out of the output terminal.
Switching Design
 Class D and E.
 In switching amplifiers the output current of the amplifying device is
pulsating.
Common Methods of Amplifier Coupling
Whether an amplifier is one of a series in a device or a single stage connected
between two other devices, there must be some way for the signal to enter and
leave the amplifier. The process of transferring energy between circuits is known
as COUPLING.
1. Direct Coupling
 The method of coupling that uses the least number of circuit elements
and that is, perhaps, the easiest to understand is direct coupling. In direct
coupling the output of one stage is connected directly to the input of the
following stage.
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UNIVERSITY OF PANGASINAN
Department of Electronics Engineering
ELECTRONICS ENGINEERING 2
Prepared by: Engr. Cezar N. Velasco Jr.
 Provides a good frequency response since no frequency-sensitive
components (inductors and capacitors) are used. The frequency response
of a circuit using direct coupling is affected only by the amplifying device
itself.
 Direct coupling has several disadvantages, however. The major problem
is the power supply requirements for direct-coupled amplifiers. Each
succeeding stage requires a higher voltage. The load and voltage divider
resistors use a large amount of power and the biasing can become very
complicated. In addition, it is difficult to match the impedance from stage
to stage with direct coupling.
 The direct-coupled amplifier is not very efficient and the losses increase
as the number of stages increase. Because of the disadvantages, direct
coupling is not used very often.
2. RC Coupling
 The most commonly used coupling in amplifiers is RC coupling.
 RC coupling does have a few disadvantages. The resistors use dc power
and so the amplifier has low efficiency. The capacitor tends to limit the
low-frequency response of the amplifier and the amplifying device itself
limits the high-frequency response.
3. Impedance Coupling
 Impedance coupling is very similar to RC coupling. The difference is the
use of an impedance device (a coil) to replace the load resistor of the first
stage.
 Used at high frequencies.
 Impedance coupling is usually not used for audio amplifiers.
4. Transformer Coupling
 No capacitor is needed because transformer action couples the signal
between the primary and secondary of Transformer.
 The inductors that make up the primary and secondary of the transformer
have very little dc resistance, so the efficiency of the amplifiers is very
high. Transformer coupling is very often used for the final output
(between the final amplifier stage and the output device) because of the
impedance-matching qualities of the transformer. The frequency response
of transformer-coupled amplifiers is limited by the inductive reactance of
the transformer just as it was limited in impedance coupling.
5. Resistive Coupling
 Connected or coupled by means of resistor
FET Amplifiers
Field-effect transistor amplifiers provide an excellent voltage gain with the
added feature of high input impedance. They are also considered low-power
consumption configurations with good frequency range and minimal size and weight.
Both JFET and depletion MOSFET devices can be used to design amplifiers having
similar voltage gains. The depletion MOSFET circuit, however, has much higher input
impedance than a similar JFET configuration.
Due to the very high input impedance, the input current is generally assumed to
be 0 µA and the current gain is an undefined quantity. While the voltage gain of an FET
amplifier is generally less than that obtained using a BJT amplifier, the FET amplifier
provides much higher input impedance than that of a BJT configuration. Output
impedance values are comparable for both BJT and FET circuits.
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UNIVERSITY OF PANGASINAN
Department of Electronics Engineering
ELECTRONICS ENGINEERING 2
Prepared by: Engr. Cezar N. Velasco Jr.
1. Common-Drain – a FET amplifier configuration in which the drain is the
grounded terminal. Providing unity gain with no inversion.
2. Common-Gate - a FET amplifier configuration in which the gate is the grounded
terminal. Providing gain with no inversion.
3. Common-Source - a FET amplifier configuration in which the source is the
grounded terminal. The most popular providing an inverted output.
Bias Confuguration
Zi
Zo
Av
High
Medium
Medium
RG
RD//rd
- gm RD//rd
High
Medium
Medium
RG
RD//rd
- gm RD//rd
Zo
Av
Ai
Phase
Shift
Fixed-bias (JFET or MOSFET)
1800
Self-bias bypassed Rs (JFET or
MOSFET)
Zi
1800
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UNIVERSITY OF PANGASINAN
Department of Electronics Engineering
ELECTRONICS ENGINEERING 2
Prepared by: Engr. Cezar N. Velasco Jr.
Self-bias Unbypassed Rs (JFET or DMOSFET)
High
RG
Voltage Divider Bias (JFET or DMOSFET)
Medium
Low
RD
gmRD
1+gmRs+ RD+Rs
rd
High
Medium
Medium
R1//R2
RD//rd
- gm RD//rd
Low
Medium
Medium
RD//rd
𝒈𝒎𝑹𝑫 + 𝑹𝑫
𝒓𝒅
180o
180o
Common Gate (JFET or D-MOSFET)
𝐫𝐝+𝐑𝐃
Rs//𝟏+𝒈𝒎𝒓𝒅
𝟏+
Source Follower (JFET or DMOSFET)
𝑹𝑫
𝒓𝒅
High
Low
Low
RG
𝟏
RD//rd//𝒈𝒎
𝒈𝒎(𝒓𝒅//𝑹𝒔)
𝟏 + 𝒈𝒎(𝒓𝒅//𝑹𝒔)
Medium
Medium
Medium
Inphase
Inphase
Drain Feedback Bias (E-MOSFET)
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UNIVERSITY OF PANGASINAN
Department of Electronics Engineering
ELECTRONICS ENGINEERING 2
Prepared by: Engr. Cezar N. Velasco Jr.
𝐑𝐅+𝐫𝐝//𝐑𝐃
𝟏+𝐠𝐦(𝐫𝐝//𝐑𝐃)
RF//RD//rd
Medium
Medium
Medium
R1//R2
RD//rd
- gm RD//rd
- gm(RF//RD//rd)
1800
Voltage Divider Bias (E-MOSFET)
1800
 The transconductance gm is determined by the ratio of the change in drain
current associated with a particular change in gate-to-source voltage in the
region of interest. The steeper the slope of the ID versus VGS curve, the greater is
the level of gm.
 The closer the region of interest to the saturation current IDSS, the greater the
transconductance.
 When VGS is one-half pinch-off value, gm is one-half the maximum value.
 When ID is one-fourth the saturation current, gm is one-half the value at
saturation.
Multistage Amplifiers
The design requirements of amplifiers normally specify an overall high voltage
gain, high input impedance, and a low output impedance. A single transistor amplifier
rarely satisfies these design requirements and multistage are often used to satisfy the
design specifications.
1. Cascaded Amplifiers
 Are stages of amplifiers connected in series, the output of the first stage
provides the input of the next stage.
 The output of one stage in a capacitively coupled amplifier is connected
to the input of the next stage via capacitor.
 In direct-coupled amplifiers, The output of one stage directly connected to
the input of the next stage
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UNIVERSITY OF PANGASINAN
Department of Electronics Engineering
ELECTRONICS ENGINEERING 2
Prepared by: Engr. Cezar N. Velasco Jr.
2. Cascoded Amplifier
 A combination or compound transistor configuration utilizing a commonemitter on the first stage feeding a common base stage. This
configuration provides high input impedance and low noise.
3. Darlington Amplifier
 Super beta transistor meaning it provides a very large current gain.
 Double emitter follower
 Beta multiplier
 Main feature is the composite
transistor acts as a single
unit with a current gain that
is the product of the current
gain of individual transistors.
4. Differential Amplifier
 A combination or compound transistor configuration designed to respond
to the difference between the two-input voltages
 Usually the input circuit for most low-noise power amplifiers and
operational amplifiers.
 Its output voltage is proportional to the difference between its two input
voltages.
 It has a high voltage gain and is directly coupled to the input voltages and
the load
Diff-amp Mode of Operation
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UNIVERSITY OF PANGASINAN
Department of Electronics Engineering
ELECTRONICS ENGINEERING 2
Prepared by: Engr. Cezar N. Velasco Jr.
1. Single-ended input
 One input is grounded and the signal voltage is applied only to the other
input.
2. Double-ended output
 Differential input.
 Two opposite-polarity (out of phase) signals are applied to the inputs.
Each input affects the outputs
3. Common-mode operation
 One of the most important aspects of operation of a diff-amp can be
seen by considering the common-mode condition where two signal
voltages of the same phase, frequency, and amplitude are applied to
the two inputs.
Common-mode rejection
 Same signals applied to both inputs, the output superimposed and
cancel, the resulting a zero output voltage.
Common-rejection ratio (CMRR)
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UNIVERSITY OF PANGASINAN
Department of Electronics Engineering
ELECTRONICS ENGINEERING 2
Prepared by: Engr. Cezar N. Velasco Jr.
 The measure of the ability of amplifiers to reject common-mode signal
 This suggests that a good measure of the diff-amp’s performance in
rejecting unwanted signals is the ratio of the differential voltage gain to
the common-mode gain.
 The higher the CMRR, the better. A very high value of CMRR means
that the differential gain is high and the common-mode gain is low
Operational Amplifiers
 Is a modular, multistage device
with differential input and entire
assembly composed on a small
silicon substrate packaged as an
IC.
 Was first introduced in 1940s
 1948, first op-amp designed using
vacuum tubes.
 It was used in the analog
computers to perform a mathematical operations
 Robert J. Widlar at Fairchild brought out the popular 741 integrated circuit
between 1964 to 1968
 Basically an excellent high gain dc amplifier.
 The input at inverting terminal results in opposite polarity (anti-phase) output.
 The input at non-inverting input terminal results in the same polarity (phase)
output.
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UNIVERSITY OF PANGASINAN
Department of Electronics Engineering
ELECTRONICS ENGINEERING 2
Prepared by: Engr. Cezar N. Velasco Jr.
Differential gain (Ad)
 Is the gain with which differential amplifies the difference between two input
signals
j
Average common level
Slew Rate
 The maximum rate at which the output voltage can change with respect to the
input.
 Represents the fastest response that an op amp can have.
Slew rate =
𝚫𝐕
𝚫𝐭
Power Supply rejection ratio
 Also called power supply sensitivity
 Defined as the ratio of the changes in input offset voltage due to the change in
supply voltage producing it, keeping other power supply coltage constant.
Features of IC 741 op-amp
1. No frequency compensation required.
2. Short circuit protection provided.
3. Offset voltage null capability.
4. Large common mode and differential voltage range.
5. No latch up.
BiFET and BiCMOS
 Some, called BiFET or BiCMOS op amps, combine an FET input stage with
bipolar transistors in the rest of the amphfier, for the advantages of extremelyhigh input resistance andnegligible input bias current, together with the high
gain of bipolar transistors.
Open Loop Configuration of Op-Amp
 The simplest possible way to use an
op-amp is in the open loop condition
 Gain is very large in open loop
condition, the output voltage Vo is
either at its positive saturation voltage
or negative saturation voltage as V1>V2
or V2>V1 respectively.
 Very small noise voltage present at the
input also gets amplified due to its
high open loop gain and op-amp gets
saturated.
 Inability of op-amp to work as a linear small signal amplifier in the open loop
condition.
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UNIVERSITY OF PANGASINAN
Department of Electronics Engineering
ELECTRONICS ENGINEERING 2
Prepared by: Engr. Cezar N. Velasco Jr.
 Rarely use in voltage comparator, zero crossing detetor.
Closed Loop Configuration of Op-Amp
 The utility of op-amp increase
considerably if it is used in a closed
loop mode.
 Closed loop is possible using
feedback (feedback resistor)
 The feedback allows to feed some
part of the output back to the input.
 The closed loop gain is much less
than the open loop gain due to the feedback resistance.
 Because of the reduced gain, the output is not driven into saturation and the
circuit behaves in a linear manner.
Advantages of Negative Feedback
1. It reduces the gain and makes it controllable.
2. It reduces the possiblity of distortion.
3. It increases the bandwidth, frequency range.
4. It increases the input resistance of the op-amp.
5. It decrease the output resistance of the op-amp.
6. It reduces the effects of temperature, power supply on the gain of the circuit.
Linear Applications of Op Amps
 Output voltage varies linearly with respect to the input voltage.
 The negative feedback is the base of linear applications.
 Voltage follower, differential amplifier, instrumentation amplifier, inverting
amplifier and non inverting amplifier.
Nonlinera Applications of Op Amps
 A feedback is provided from the output to the input terminal, the feedback may
be provided to the inverting input terminal using nonlinear elements like diodes,
transistors.
 Nonlinear applications are rectifiers, comparators, clampers, limiters, schmitt
trigger circuit.
Inverting Amplifier
 Provides a phase shift
between input and output.
 The input impedance is R1.
Gain (Av) =
𝑽𝒐
𝑽𝒊
=−
of
180o
𝑹𝒇
𝑹𝟏
Observations:
1. The output is inverted with respect to input, which is indicated by minus sign.
2. The voltage gain is independent of open loop gain of the op-amp, which is
assumed to be large.
3. The voltage gain depends on the ratio of the two resistances. Hence selecting Rf
and R1, the required value of gain can be easily obtained.
4. If Rf > R1, the gain is greater than 1.
5. If Rf < R1, the gain is less than 1.
6. If Rf = R1, the gain is unity.
i. Thus the output voltage can be greater than, less than or equal to
the input voltage, in magnitude.
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UNIVERSITY OF PANGASINAN
Department of Electronics Engineering
ELECTRONICS ENGINEERING 2
Prepared by: Engr. Cezar N. Velasco Jr.
7. If the ratio of Rf and R1 is K which is other than one, the circuit is called scale
changer while for Rf / R1 = 1 it is called phase inverter.
8. The closed loop gain is denoted as AVF or AVCL i.e. gain with feedback.
Noninverting Amplifier
 An amplifier which amplifies the input
without producing any phase shift
betweeninput and output.
AVF =
𝑽𝒐
𝑽𝒊
= 𝟏+
𝑹𝒇
𝑹𝟏
 The positive sign indicate that there is no phase shift between input and output.
 The voltage gain is always greater than 1.
 The input impedance is extremely large.
Observations:
1. The voltage gasin is always greater than 1.
2. The voltage gain is positive indicating that for a.c. input, the output and input
are in phase while for d.c. input, the output polarity is same as that of input.
3. The voltage gain is independent of open loop gain of op-amp, but depends only
on the two resistance values.
4. The desired voltage gain can be obtained by selecting proper values of R f and
R1.
2
K
Voltage Follower
 The output voltage is equal to the input voltage.
 It is also called source follower or emitter
follower
 Unity gain amplifier
 Buffer amplifier
 Isolation amplifier
Advantages of Voltage Follower
1. Very large input resistance, of the order of MΩ.
2. Low output impedance, almost zero. Hence it can be used to connect high
impedance source to a low impedance load, as a buffer.
3. It has large bandwidth.
4. Low output offset error because the input errors are not amplified.
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UNIVERSITY OF PANGASINAN
Department of Electronics Engineering
ELECTRONICS ENGINEERING 2
Prepared by: Engr. Cezar N. Velasco Jr.
Summer/summing amplifier or Adder Circuit
 The output level is determined by adding the input signals together (although
the output signal will be of opposite polarity compared to the sum of the input
signals).
 Depending upon the sign of the output, the summer circuits are classified as
inverting and noninverting summer.
Inverting Summer
All the input signals to be added are applied
to the invrting input terminal of the op-amp.
Noninverting Summing Amplifier
There is no phase inversion
Difference Amplifier (Subtractor)
 A difference amplifier will produce an output based on the difference between
the input signals.
Integrator
 The output voltage is the integration of the input
voltage.
 The integator circuit can be obtained without
using active devices like op-amp, tansistors etc.,
it is called passive integator.
 An integrator that uses an active devices like op-amp is called active integrator.
Differentiator
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UNIVERSITY OF PANGASINAN
Department of Electronics Engineering
ELECTRONICS ENGINEERING 2
Prepared by: Engr. Cezar N. Velasco Jr.
 Produces the differentiation of the input voltage at its
output
 The differentiator which does not use any active device
is called passive differentiator.
 The differentiator that uses active devices is called active differentiator.
 Produces an output that is proportional to the rate of change of the input
voltage.
Applications of Practical Differentiator
1. In the wave shaping circuits to detect the high frequency component in the input
signal.
2. As a rate-of-change detector in the FM demodulators.
3. The differentiator is avoided in the analog computers.
Comparators
 Is a type of op-amp circuit that compares two input voltages and produces an
output in either of two states indicating the greater than or less than relationship
of the inputs.
 Zero-level Detection, application of op-amp used as a comparator is to
determine whwn an input voltage exceeds a certain level.
 Nonzero-level detector, is to detect positive and negative voltages by
connceting a fixed reference voltage source to the inverting input.
Hysteresis
 A techniques incorporating positive feedback to reduce noise effects.
 A comparator with hysteresis is sometimes known as Schmitt trigger.
To limit the output voltage levels of a comparator to a value less than that
provided by the saturated op-amp, a single zener diode can be used, and this
process of limiting the output range is called bounding.
The Comparator with positive feedback for
hysteresis. The upper trigger point and
lower trigger point.
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UNIVERSITY OF PANGASINAN
Department of Electronics Engineering
ELECTRONICS ENGINEERING 2
Prepared by: Engr. Cezar N. Velasco Jr.
Other Applications of Comparator
1. Over-temperature Sensing
 The circuit consists of a wheatstone bridge with the op-amp used to
detect when the bridge is balanced
2. Analog-to-digital conversion
 A common interfacing process often used when a linear analog system
must provide inputs to a digital system.
Special Purpose Op-amp Circuits
1. Instrumentation Amplifier
 A
differential
voltage-gain
device that amplifies the
difference
between
the
voltages existing at its two
input terminals.
 Main purpose is to amplify
small signals that are riding on
large common-mode voltages.
 High input impedance, high
common-mode rejection, low
output offset, and low output impedance.
 Made up of three op-amps and several resistors and the voltage gain is
set with an external resistor.
 AD622, integrated circuit instrumentation amplifier.
2. Isolation Amplifier
 Provides between input
and output
 It
is
used
for
the
protection of human life or
sensitive equipment in
those applications where
hazardous
power-line
leakage or high-voltage
transients are possible.
 Consists
of
two
electrically
isolated
stages.
 It uses optical coupling or
transformer coupling to
provide isolation between
the stages.
 IS0124 is an integrated circuit isolation amplifier
 Burr-Brown 3656KG is an example of an isolation amplifier that uses
transformer coupling.
 Require no common grounds between transducer and the processing
circuits where interfacing to sensitive equipment.
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UNIVERSITY OF PANGASINAN
Department of Electronics Engineering
ELECTRONICS ENGINEERING 2
Prepared by: Engr. Cezar N. Velasco Jr.
3. Operational Transconductance Amplifiers (OTAs)
 Primarily a voltage-to-current amplifier in
which the output current equals the gain
times the input voltage.
 Transconductance is the ratio of the output
current to the input voltage.
 Two
OTA
Applications:
Amplitude
Modulator and Schmitt Trigger.
4. Log and Antilog Amplifiers
 Are
used
in
applications
that
require
compression of analog input data, linearization of
transducers that have exponential outputs, and analog multiplication and
division.
 A logarithmic amplifier produces an
output that is proportional to the
logarithm of the input, and the antilog
amplifier take the antilog or inverse log
of the input.
5. Constant-Current Source
 Delivers a load current that remains constant when the load resistance
changes.
6. Current-to-voltage converter
 Converts a variable input current to a proportional output voltage.
7. Voltage-to-current Converter
 Provide an output load current that is controlled by an input voltage.
8. Peak Detector
 Op-amp is used as a comparator.
 Used to detect the peak of the input voltage and store the peak voltage on
a capacitor.
Video Amplifier
 Used to amplify signals that represent video information.
 Some amplifiers should be "flat" across a band of frequencies. In other words,
for every frequency within the band, the amplifier should have equal gain (equal
response). For frequencies outside the band, the amplifier gain will be much
lower.
 For other amplifiers, the desired frequency response is different. For example,
perhaps the amplifier should have high gain at two frequencies and low gain for
all other frequencies. The frequency-response curve for this type of amplifier
would show two "peaks." In other amplifiers the frequency-response curve will
have one peak indicating high gain at one frequency and lower gain at all others.
High-frequency Compensation for Video Amplifiers
 If the frequency-response range of an audio amplifier must be extended to 6
megahertz (6 MHz) for use as a video amplifier, some means must be found to
overcome the limitations of the audio amplifier. As you have seen, the
capacitance of an amplifier circuit and the interelectrode capacitance of the
transistor (or electronic tube) cause the higher frequency response to be limited.
18
UNIVERSITY OF PANGASINAN
Department of Electronics Engineering
ELECTRONICS ENGINEERING 2
Prepared by: Engr. Cezar N. Velasco Jr.
 Since frequency affects capacitive reactance and inductive reactance in
opposite ways, and since it is the capacitive reactance that causes the problem
with high-frequency response, inductors are added to an amplifier circuit to
improve the high-frequency response.
 Inductors (coils), when used for high-frequency compensation, are called
PEAKING COILS. Peaking coils can be added to a circuit so they are in series
with the output signal path or in parallel to the output signal path. Instead of
only in series or parallel, a combination of peaking coils in series and parallel
with the output signal path can also be used for high-frequency compensation.
Low-frequency Compensation for Video Amplifiers
 The culprit in low-frequency response is capacitance (or capacitive reactance).
But this time the problem is the coupling capacitor between the stages.
 This high reactance limits the amount of output signal that is coupled to the next
stage. In addition, the RC network of the coupling capacitor and the signaldeveloping resistor of the next stage cause a phase shift in the output signal.
 Both of these problems (poor low-frequency response and phase shift) can be
solved by adding a parallel RC network in series with the load resistor.
Radio-Frequency Amplifier
 Are used to amplify signals between 10 kilohertz (10 kHz) and 100,000
megahertz (100,000 MHz) (not this entire band of frequencies, but any
band of frequencies within these limits). Therefore, any device that uses
frequencies between 10 kilohertz and 100,000 megahertz will most likely
use an rf amplifier.
 The parallel LC circuit used as a frequency-determining network is called
a TUNED CIRCUIT. This circuit is "tuned" to give the proper response at
the desired frequency by selecting the proper values of inductance and
capacitance.
 One way of "broadening" the bandpass of a tuned circuit is to use a
swamping resistor. This is similar to the use of the swamping resistor that
was shown with the series peaking coil in a video amplifier.
 One of these problems is the losses that can occur in a transformer at
these high frequencies. Another problem is with interelectrode
capacitance in the transistor. The process of overcoming these problems
is known as COMPENSATION.
 The problem of interelectrode capacitance in the transistor of an rf
amplifier is solved by NEUTRALIZATION. Neutralization is the process of
counteracting or "neutralizing" the effects of interelectrode capacitance.
Magnetic Amplifier
 The magnetic amplifier does not amplify magnetism, but uses electromagnetism
to amplify a signal. It is a power amplifier with a very limited frequency
response. Technically, it falls into the classification of an audio amplifier; but,
since the frequency response is normally limited to 100 hertz and below, the
magnetic amplifier is more correctly called a low-frequency amplifier.
 A magnetic amplifier uses a changing inductance to control the power delivered
to a load.
 The magnetic amplifier has certain advantages over other types of amplifiers.
These include (1) high efficiency (up to 90 percent); (2) reliability (long life,
freedom from maintenance, reduction of spare parts inventory); (3) ruggedness
(shock and vibration resistance, high overload capability, freedom from effects
of moisture); and (4) no warm-up time. The magnetic amplifier has no moving
parts and can be hermetically sealed within a case similar to the conventional
dry-type transformer.
19
UNIVERSITY OF PANGASINAN
Department of Electronics Engineering
ELECTRONICS ENGINEERING 2
Prepared by: Engr. Cezar N. Velasco Jr.
Voltage-feedback Amplifiers (VFAs)
 With this type of amplifier, the feedback network feeds back a voltage to the
inverting input, and for this reason such op amps are sometimes called
voltage-feedback amplifiers. Although this type can have a gain bandwidth
product (GB) of several gigahertz, its closed-loop bandwidth is usually
limited by stability requirements to lower values.
Current-feedback amplifiers (CFAs)
 A different circuit topology is used in circuits intended for wider bandwidth at
lower closed loop gain. These are known as current-feedback op amps.
 In the CFA, the high load impedances lead to a high output voltage for a
small input current, and the ratio of output voltage to input current /B is very
high. This ratio has dimensions of ohms, and is known as the forward
transimpedance, Zf.
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