Description
R1
Vi1
R3
Vi2
R2
+5V
2
7
1
V+
6
N1
N2
V−
5
3
4
R4
−5V
Vo
Figure 2: Differential amplifier circuit
c Department of Electrical and Computer Engineering, Texas A&M University
1
This expression shows that the circuit amplifies the difference between the two input signals Vi2 Vi1 and rejects the common mode input signals (Vo = 0 if Vi1 = Vi2). Therefore, the differential amplifier can be used in a very noisy environment to reject common noise that appears at both inputs. When the same signal is applied to both inputs, the voltage gain is defined as common-mode gain (ACM ), which is zero for an ideal differential amplifier. The common-mode rejection ratio is defined as,
CMRR =
ADM (Differential-mode gain)
(3)
ACM (Common-mode gain)
Substituting ACM = 0 to the above expression, CMRR for an ideal differential amplifier becomes infinite. In practice, resistors have a tolerance of typically 5%, and the common-mode gain will not be zero, resulting in finite CMRR.
Instrumentation Amplifier
The instrumentation amplifier is a differential amplifier that has high input impedance and the capability of gain adjustment through the variation of a single resistor. A typical instrumentation amplifier is shown in Fig. 3.
+5V
3
7
1
V+
N1
6
Vi1
V−
N2
5
2
4
−5V
+5V
2
7
1
V+
N1
6
N2
V−
3
4
5
Vi2
−5V
R
R
Rgain
R
R
R
+5V
2
7
1
V+
N1
6
N2
V−
5
3
4
−5V
R
Vo
Figure 3: Typical instrumentation amplifier circuit
The voltage drop across Rgain is equal equal to the voltage difference of the two input signals. Therefore, the current through Rgain caused by the voltage drop must flow through the two R resistors above and below Rgain. The output voltage can be calculated as
Vo = 1 +
2R
(Vi2 Vi1)
(4)
Rgain
Though this configuration looks cumbersome to build a differential amplifier, the circuit has several properties that make it very attractive. It presents high input impedance at both terminals because the inputs connect into non-inverting terminals. Also a single resistor Rgain can be used to adjust the voltage gain.
Calculations
1.
For the summing amplifier in Fig. 1, find R1 and R2 to have Vo =
(Vi1 + 2Vi2), if R3 = 15k .
2.
For the differential amplifier in Fig. 2, find R1 to have Vo = Vi2
Vi1, if R2 = R3 = R4 = 10k .
3.
For the instrumentation amplifier in Fig. 3, find R to have Vo = 3(Vi2 Vi1), if Rgain = 1k .
4.
For each circuit, find Vo if Vi1 = 0.2 sin(2 1000t) and Vi2 = 0.3V .
2
Simulations
For all simulations, provide screenshots showing the schematics and the plots with the simulated values prop-erly labeled.
1. Draw the schematics for the circuits in Figs. 1, 2, and 3 with the calculated component values using the UA741 opamp model.
2. Apply the inputs Vi1 and output voltages
• 0.2 sin(2 1000t) and Vi2 = 0.3V , and obtain the time-domain waveforms for the input using transient simulation. Confirm that the circuits operate as designed.
Measurements
For all measurements, provide screenshots showing the plots with the measured values properly labeled.
Summing Amplifier
1. Build the circuit in Fig. 1 with the simulated component values.
2. Apply the inputs Vi1 = 0.2 sin(2 1000t) and Vi2 = 0.3V , and obtain the time-domain waveforms for the input and the output voltages using the scope to confirm that the circuit is a summing amplifier.
3. Raise the DC input voltage Vi2 until clipping at the output is observed.
Differential Amplifier
1. Build the circuit in Fig. 2 with the simulated component values.
2. Apply the inputs Vi1 = 0.2 sin(2 1000t) and Vi2 = 0.3V , and obtain the time-domain waveforms for the input and the output voltages using the scope to confirm that the circuit is a differential amplifier.
3. Apply Vi1 = 0.2 sin(2 1000t) and connect Vi2 to ground. Measure ADM = Vo =Vi .
4. Apply Vi1 = Vi2 = 0.2 sin(2 1000t). Measure ACM = Vo =Vi .
5. Calculate the common-mode rejection ratio (CMRR).
Instrumentation Amplifier
1. Build the circuit in Fig. 3 with the simulated component values.
2. Apply the inputs Vi1 = 0.2 sin(2 1000t) and Vi2 = 0.3V , and obtain the time-domain waveforms for the input and the output voltages using the scope to confirm that the circuit is a instrumentation amplifier.
Report
1. Include calculations, schematics, simulation plots, and measurement plots.
2. Prepare a table showing calculated, simulated and measured results.
3. Compare the results and comment on the differences.
Demonstration
1. Build the circuits in Figs. 2 and 3 on your breadboard and bring it to your lab session.
2. Your name and UIN must be written on the side of your breadboard.
3. Submit your report to your TA at the beginning of your lab session.
4. For the differential amplifier in Fig. 2:
Show the time-domain waveforms with Vi1 = 0.2 sin(2 1000t) and Vi2 = 0.3V .
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Measure Adm with Vi1 = 0.2 sin(2 1000t) and Vi2 = 0. Measure Acm with Vi1 = Vi2 = 0.2 sin(2 1000t).
Calculate CMRR
5. For the instrumentation amplifier in Fig. 3:
Show the time-domain waveforms with Vi1 = 0.2 sin(2 1000t) and Vi2 = 0.3V .
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