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UNIVERSITY OF UTAH
ELECTRICAL AND COMPUTER ENGINEERING DEPARTMENT
ECE 3110 LABORATORY EXPERIMENT NO. 1
CURRENT MIRRORS AND DIFFERENTIAL AMPLIFIERS
Introduction
This lab covers basic bipolar transistor current mirrors and bipolar transistor differential amplifiers, both basic building blocks for analog integrated circuits. We will use 2N3904 NPN transistors during this lab. The pinout diagrams of discrete transistors are shown below.
B E
E B C C
E
B
E C
C
B
NPN PNP
plastic
package
metal
package
Figure 1. Pin-out diagram of discrete bipolar transistors
In this experiment, you will construct and characterize the following circuits:
- A simple BJT current mirror (text, pp. 567-569).
- A "Widlar" current source (text, pp. 653-655).
- A simple differential amplifier (text, section 7.3).
- An improved differential amplifier which uses a current mirror to provide the bias current.
Experiment I. Current Mirror
Build the circuit shown in Figure 2 using two NPN transistors. The 50k resistor is a potentiometer and the voltage source on the output is a variable DC source.
I o
1 2
ref
+6 V
0-10 V
Q Q
50 kΩ
4.7 kΩ
I
Vo
Rout
Figure 2.
For the settings: 5kΩ, 25kΩ, and 50kΩ of the 50-kΩ potentiometer, do the following:
a. Measure I (^) ref and Vbe1. Vbe1 is the base-emitter voltage of Q 1.
b. Measure I (^) o while varying Vo from 0 to 10 volts, in 500mV increments. Don’t forget to check the multi-meter mode before you measure a current or voltage, if it is set incorrectly you will blow a fuse. How does the mismatch between the two transistors affect the current mirror’s gain (Io / I (^) ref )? “Mismatch” just refers to the fact that two discrete transistors often have slightly different
As seen from this equation, RE can be chosen to give the desired I (^) o so long as I (^) o < I (^) ref.
Assume I (^) ref = 1 mA and calculate the value of RE required to give I (^) o = 50 microamps (note that a closed form solution doesn’t exist, try some values and iterate until the both sides are approximately equal).
Construct the circuit in Figure 3 using a value of RE close enough to the calculated RE that I (^) o = 50 microamps ± 5%. What value of RE did you choose? Plug the value back into the equation and report the expected I (^) o.
Set I (^) ref = 1.0mA by adjusting the 50-kΩ potentiometer. Vary Vo from 0 to +10 Volts in 500mV increments and record I (^) o. Use the highest sensitivity meter and setting available because the current won’t change very much and will make the output resistance calculation inaccurate or impossible. Using MATLAB, plot I (^) o vs. Vo. From the measured data, calculate the small-signal output resistance, Ro , at I (^) o about equal to 50 μA. Is the output resistance of the Widlar current mirror higher or lower than a standard current mirror? Is this expected?
III. Differential Amplifiers
Build the differential amplifier circuit shown in Figure 4.
+10 V
v o
v
Q Q
-10 V
v
10 K10 K10 K10 K
10 K 10 K
Figure 4. Basic differential amplifier.
You are to measure the common mode voltage gain, ACM, and the differential mode voltage gain, Ad , of the circuit in Fig. 4. (See text, pp. 713-717.)
a. Common Mode Gain
Connect a 1-kHz sinusoidal signal of 1 Volt peak-to-peak to both inputs, as shown in Figure 5. Note that the differential amplifier itself has been represented as a block. Remember to set the HP 33120A to high-impedance (“HI-Z”) load mode for the following experiments, or your signal amplitudes will be off by a factor of two!
o
vi
Diff
– Amp^ v
20K
50K
v+
v-
Figure 6. Measurement set-up for differential mode voltage gain.
Adjust the 50-kΩ potentiometer until v+ and v (^) – are exactly equal in magnitude. To do this, connect a 1-kHz, 1-volt sinusoidal signal to vi and feed v+ into one input of your oscilloscope and v (^) – into the other. Set the scope to sum the two channels (note that v+ and v (^) – have opposite signs, so the difference of the two voltages is actually being displayed). Adjust the range of the scope to insure that the difference signal is as close to zero as possible.
Now reduce v (^) i until vo is sinusoidal (for no distortion, again you may need to build the voltage divider to attenuate the input signal). The adjusted vi will be in the range of 50 millivolts or so. Measure vi and v (^) o. The measurement of vi requires some care, for noise can contaminate such a small signal. Compute the differential mode gain (vo /vi).
Use the differential gain and your previously computed common mode gain to calculate CMRRdB (see pg. 716 in text). Build the circuit shown in Figure 7, using a current source in the common emitter lead to improve the CMRR.
Compute the value of R which will result in the same emitter bias currents for Q 1 and Q 2 as those used in the circuit of Fig. 4 (with v+ = v– = 0). Assume β and r (^) o are infinite for this calculation. Use this value of R in your circuit. Measure the values of common and differential mode voltage gains using the same techniques as above. Calculate and report the common mode rejection ratio. Is it higher? If so, why? +10 V
v (^) o
v
Q Q
-10 V
v
10 K 10 K
(^1 )
Q
R
3 Q^4
Figure 7. Improved differential amplifier.
Report: Answer these questions in your lab notebook.
- Using your measured value of the output resistance Ro , of the simple current mirror of Figure 2, part 1, calculate the Early voltage VA of the 2N3904. What significance does Ro have in the design of differential amplifiers?
- a. Compare the measured values of ACM, Ad , and CMRRdB for both of the differential amplifiers you built (resistor tail current and current mirror tail current). What are the major differences and why do they occur? Which one is more stable?
