Phys 3610/6610 Lab 20
Student:
TA:
Multi-Transistor Circuits: Current Mirror and Differential Pair
This lab will make use of the CA 3046 transistor array. Due to the close matching of transistor
parameters for transistors in a single IC it is possible to build circuits that are impossible to build
with discrete transistors.
Two of the transistors on your CA 3046 are to be connected as an emitter-coupled or differential
pair, and another two or three will be used to form a current mirror to supply the emitter current.
Use the +5 V and the -12 V supplies, as it would exceed the collector substrate rating to use
the ±12 V supplies. The substrate in the IC (pin 13) must be connected to the -12 V supply. The
resistor from ground to the current mirror should be chosen such that the common emitter current
is 2 mA. Again: document all your calculations. We suggest collector resistors of 1 kΩ, but you
may choose other values. For all except the last part of the lab take the output from one of the
collectors.
Task 1: Use a voltage divider to create a 0 to 1 V square wave input signal at 1 kHz. Use this input
to measure the common mode gain of your circuit. The common mode gain is the voltage gain
Gcm = Vout /Vin of the amplifier when both differential inputs are connected together (common)
and are driven by Vin .
Task 2: Ground one input and apply a 0 to < 25 mV square wave input signal at 1 kHz to the
other input. Measure the circuit gain GG = Vout /Vin to get the common mode rejection ratio:
CMRR = GG /Gcm
Does this remind you of another circuit element that is on a single silicon chip and shows similar
behavior?
Task 3: With the inputs connected as in task 2, disconnect the common emitter from ground and
vary the common emitter current at a 60 Hz rate between 2 mA and about 2.83 mA [= 2 mA ×
(17/12) = 2 mA × (12+5)/12, if there ever was a hint...]. Observe and record the output at the
differential pair collectors.
Task 4: Use your LM 741 op-amp in a differential amplifier configuration to “look at” the difference in the outputs from the two collectors of the setup used in task 3. Observe and record the
op-amp output. Does the output look like you expected?
-1-