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Differential
Amplifiers
FERNANDO VICTOR V. DE VERA, ECE, M.TECH
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Differential Amplifiers
An amplifier that operates with two possible inputs and two
possible outputs.
It operates on the difference between two inputs.
The input circuit of most low-noise power amplifiers and
operational amplifiers.
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Common Emitter and Common
Base Configuration
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138
The Basic Differential Amplifier
The combination of
Common-Emitter and
Common-Base
Configuration
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Differential Amplifier Operation
𝑉 =𝐴× 𝑉 −𝑉
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140
Differential Amplifier Operation
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The Typical Differential Amplifiers
The typical Differential
amplifiers consists of two
transistors, with two
possible inputs and two
possible outputs.
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142
Single-Input, Single-Output,
Differential Amplifier
A differential amplifier
that has one input and
one output, the other
input is grounded and
the other output is not
used.
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Single-Input, Differential-Output,
Differential Amplifier
With single input, it
produces two outputs
out-of-phase with each
other, therefore
producing a differential
output.
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144
Differential-Input, Differential
Output, Differential Amplifier
With two-inputs, two outof-phase outputs
producing a differential
output.
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Assignment
For a BJT differential amplifier, show a step-by-step
explanation of the derivation of the formula for the
following:
A. Voltage Gain
B. Current Gain
C. Input Impedance
D. Output Impedance
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Operational
Amplifiers
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Operational Amplifiers
It is an amplifier which is
designed to be used with
other components to
perform either
computing functions or
some type of transfer
operation such as
filtering.
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Characteristic of Op-Amp
Very High Gain
◦ (A=∞)
Very High Input Impedance
◦ (Zin = ∞ ohms)
Very Low Output Impedance
◦ (Zout = 0 ohms)
Very Wide Bandwidth
◦ (Bw = ∞ Hz)
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Block Diagram of an OP-AMP
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OP-AMP Input Signal Modes
SINGLE-ENDED Differential Mode
DOUBLE-ENDED Differential Mode
COMMON MODE
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Common Mode and its Rejection
Common Mode
COMMON-MODE REJECTION
◦ Common-mode signals (noise) will not appear on the output and
distort the desired signal.
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152
Op-Amp Parameters
Common-Mode Input Voltage Range
Input Offset Voltage
Input Bias Current
Input Impedance
Input Offset Current
Slew Rate
Common-Mode Rejection Ratio
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Common-Mode Input Voltage
Range
It is the range of input voltages which when applied to both
input, will not cause clipping or other output distortion.
Voltage range of +/- 10V with dc supply voltage of +/- 15V is
common to Op-amps.
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Input Offset Voltage
It is the differential dc voltage required between the inputs
to force the output to zero volts.
Typical value of Input Offset Voltage is 2mV or less. In ideal
case, it is 0V.
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Input Currents
INPUT BIAS CURRENT
◦ It is the dc current required by the inputs of
the amplifier to properly operate the first
stage.
◦ It is the average of both input currents.
INPUT OFFSET CURRENT
◦ It the difference of the input bias current,
expressed as absolute value.
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156
Input Impedance
DIFFERENTIAL INPUT IMPEDANCE – is the
total resistance between the inverting
and non-inverting inputs.
COMMON-MODE INPUT IMPEDANCE – is
the resistance between each input and
ground and is measured by determining
the change in bias current for a given
change in common mode input voltage.
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Slew Rate
SLEW RATE
◦ The maximum rate of change of the
output voltage in response to a step
input voltage.
◦ The slew rate is dependent upon the
high-frequency response of the
amplifier stage within the op-amp.
◦ It is measured as a voltage change in
a given time - typically V/µs or V/ms.
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158
Slew Rate
𝑆𝑅 = 2𝜋𝑓 𝑉
Where:
f = the highest signal frequency, Hz
V = the maximum peak voltage of the signal.
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or atTime
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Guessing
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Common-Mode Rejection Ratio
(CMRR)
The measure of an amplifier’s ability to reject common-mode
signals.
It is the ratio of the open-loop gain of and the commonmode gain of the Op-amp.
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160
OP-AMP CMRR
Where:
Ad = Differential Gain
Ac = Common-mode Gain
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orProduct
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AC Considerations-Unity
Gain BW
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OP-AMP Electrical Characteristics
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OP-AMP Circuit Analysis
1. The Voltage Across the input
terminals is ZERO
0A
a
𝑉 −𝑉 =0
𝑉 =𝑉
2. The current entering the
input terminals are ZERO
𝐼 =𝐼 =0
0V
0A
b
Negative Feedback
3. Apply Ohms Law
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OP-AMP Circuit Configurations
Inverting
Subtractor
Non-Inverting
Integrator
Voltage Follower
Differentiator
Summing
Comparator
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164
Inverting Configuration
The input signal is
connected to the
inverting input of the OpAmp.
The output signal is
inverted with respect to
the input.
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Inverting Amplifier
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Non-InvertingorAmplifier
166
Non-Inverting Configuration
The input signal is
directly connected to the
inverting input of the OpAmp.
The output is In-phase
with the input.
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Non-Inverting Amplifier
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VOLTAGEorFollower
168
Voltage Follower (Unity Follower)
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SummingorAmplifier
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Summing Amplifier
The output level is
determined by adding
the input signals
together although the
output signal will be the
opposite polarity
compared to the sum of
the input signals.
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170
Summing Amplifier
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SUBTRACTOR
or DIFFERENCINGorAmplifier
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Subtractor or Differencing Amp
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INTEGRATOR
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Integrator Circuit
This produces an output
voltage which is proportional
to the integral of the input
voltage.
The magnitude of the output
signal is determined by the
length of time a voltage is
present at its input as the
current through the feedback
loop charges or discharges the
capacitor as the required
negative feedback occurs
through the capacitor.
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Integrator Circuit
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DIFFERENTIATOR
174
Differentiator Circuit
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Differentiator Circuit
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to OP-AMP Electrical Characteristics
176
OP-AMP as a Comparator
A comparator compares
two input voltages and
produces an output that
is always at either one of
two states, indicating the
greater or less than
relationship between the
inputs.
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OP-AMP as a COMPARATOR
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178
Assignment
A. Discuss this circuit,
explain its properties
and its application.
B. Show a step-by-step
explanation of the
derivation of the
formula for V_out.
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