Jeffrey Chen
Section 001H
Oct 29 2012
Op Amp Design Lab
Task 1: Basic Inverting Amplifier
Objective:
Design an op amp circuit that inverts and amplifies a mono signal from a dynamic microphone.
Schematic of Networks:
Comments on Implementation:
Voltage sources of ±15V are implemented using the 20V output of an Agilent Power
Supply. V1 is implemented using an Agilent Function Generator in “high Z” mode. All
calculations assume that the op amp is operating in linear mode and that it is ideal.
Resistor values can be any multiple of the ones chosen while maintaining the ratio
between the two.
Theory of Operation:
The op amp linearly amplifies a signal within its operating specifications. When input is on the
negative terminal, it tries to match the two terminal voltages by outputting and amplified signal
and uses a negative feedback loop to control the amount of amplification. The amount of
amplification is called the gain and is negative for input on the negative terminal and positive for
input on the positive terminal.
Measurements: None
Calculations:
𝐺𝑎𝑖𝑛 =
𝑉𝑓
8
= − = −80
𝑉𝑠
.1
𝐺𝑎𝑖𝑛 = −
𝑅2
= −80
𝑅1
𝑅2 = 80𝑅1
𝑅1 = 3𝑘Ω ∴ 𝑅2 = 240𝑘Ω
% 𝑒𝑟𝑟𝑜𝑟 =
=
(𝑒𝑠𝑡𝑖𝑚𝑎𝑡𝑒 − 𝑎𝑐𝑡𝑢𝑎𝑙)
𝑎𝑐𝑡𝑢𝑎𝑙
(16 − 15.78)
15.78
× 100%.
× 100% = 1.39%
Oscilloscope Captures:
Observations/Conclusions:
The op amp circuit is fairly easy to design and build. With minimal parts, an input signal can be
amplified linearly to a certain degree specified by the voltage inputs on the op amp as well as the op
amp specifications. Although no op amp is truly ideal, realistically, most come fairly close which is why
the implementation comes so close to the theoretical calculations. The weak point in the circuit is
definitely the resistor values. Because the gain is so high in this implementation, small changes in
resistor values due to manufacturing tolerance can make a significant difference in the operation of the
circuit.
Task 2: Weighted Summing Amplifier
Objective:
Design an op amp circuit that takes an unbalanced stereo signal and sums, inverts, balances, and
amplifies it.
Schematic of Networks:
Comments on Implementation: See previous task.
Theory of Operation:
The output is a product of constructive interference and varied gain on the two inputs. The
principle of superposition applies because the circuit is assumed to be operating linearly.
Therefore, the two signals are added together and amplified according to the resistors directly
in series with them.
Measurements: None
Calculations:
𝐺𝑎𝑖𝑛𝑅𝑖𝑔ℎ𝑡 =
𝑉𝑓
𝑉𝑠
𝐺𝑎𝑖𝑛𝑅𝑖𝑔ℎ𝑡 = −
4
= − .1 = −40 𝐺𝑎𝑖𝑛𝐿𝑒𝑓𝑡 =
𝑅2
𝑅1
= −40
𝑉𝑓
𝑉𝑠
𝐺𝑎𝑖𝑛𝐿𝑒𝑓𝑡 = −
4
= − .25 = −16
𝑅2
𝑅3
= −16
𝑅2 = 40𝑅1 = 16𝑅3
𝑅2 = 80𝑘Ω ∴ 𝑅1 = 2𝑘Ω AND R 3 = 5𝑘Ω
Oscilloscope Captures:
Observations/Conclusions:
Using the principles of superposition, it is relatively simple to calculate resistor values needed to
achieve the desired output. As with the previous task, the resistor values are arbitrary as long as the
ratios remain the same and the operation is very close to ideal. This task illustrates the flexibility of
operation of op amps. This type of summing amplifier can be used in a number of audio applications for
instance the amplifier on a subwoofer which requires a mono signal or a down-mixer for editing
purposes.
Task 3: Two Channel Mixer with Balanced Inputs
Objective:
Design and build an op amp circuit that mixes two inputs with variable gain to have a set voltage
swing.
Schematic of Networks:
Comments on Implementation:
The potentiometers have one end disconnected. The wiper terminal is connected to the
op amp input to allow for a variable resistance.
Measurements: None
Calculations:
Gain of the op amp in lower limit:
Gain =
Gain =
−R 7
R1 + R 3
Vout
0.4V
=−
= − − 0.4Vpkpk
2 ∗ Vin
1V
Gain of the op amp in higher limit:
Gain =
−R 7
R3
Vout
16V
=−
= −16Vpkpk
2 ∗ Vin
1V
Assuming 20kΩ potentiometers and solving for the resistors when R 3 = R 4 yieldsR 3 = R 4 =
.513kΩ and R 7 = 8.205 kΩ.
Gain =
Oscilloscope Captures:
Wiper set to 0%
Wiper set to 100%
Observations/Conclusions:
Unlike the previous circuits, there is only one correct resistor value for each resistor used in the
circuit. While only showing the maximum and minimum voltage, the potentiometer allows for virtually
infinite adjustment within the designed range. Introduction of another resistor value also introduces
another source of error. The voltages were slightly off even though the calculations were absolute
indicating that potentiometers are not as accurate as fixed value resistors. Again a very obvious
application is in audio such as on a DJ turntable. Two circuits could be used in conjunction to mix stereo
signals.
Task 4: Level-shifting Amplifier
Objective:
Design a circuit that amplifies a microphone signal while removing an inherent DC offset
simultaneously.
Schematic of Networks:
R4
80kΩ
XSC1
VEE
-15V
C2
Tektronix
P
G
1 2 3 4
T
10nF
V1
4
R1
U1A
2
3kΩ
1
0.3 Vpk
440 Hz
0°
3
8
LF412CN
C3
R2
R3 84kΩ
16kΩ
10nF
VCC
15V
Comments on Implementation:
Voltage division is used to achieve the correct level shift voltage on the positive input
terminal of the op amp because there is only a fixed 15V power line.
Measurements: None
Calculations:
Gain =
−R4
R1
Gain = 1 +
for the inverting terminal
R4
R1
for the noninverting termnal
Oscilloscope Captures:
Observations/Conclusions:
Due to the nature of the gain, the resistors used in voltage division are dictated by the ones
chosen for the negative terminal gain. Because of this, it is very hard to design the circuit so that all the
numbers work out in rational whole numbers. There is a slight difference in the projected values and the
implemented values for this very reason. There are many reasons for removing a DC offset on an input
signal. It can be used to prevent clipping and to level match the signal with the reference signal level.
Task 5