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Activity 12: BJT Differential pair


To investigate the simple differential amplifier using NPN transistors.


As in all the ALM labs we use the following terminology when referring to the connections to the M1000 connector and configuring the hardware. The green shaded rectangles indicate connections to the M1000 analog I/O connector. The analog I/O channel pins are referred to as CA and CB. When configured to force voltage / measure current -V is added as in CA-V or when configured to force current / measure voltage -I is added as in CA-I. When a channel is configured in the high impedance mode to only measure voltage -H is added as CA-H.

In this Lab you will be using both of the channels of the ALM1000 in the split Input / Output mode. CB-Out is used to denote the connection to the Output pin and CB-In is used to denote the Input pin on the (expanded) 8 pin connector.

Scope traces are similarly referred to by channel and voltage / current. Such as CA-V , CB-V for the voltage waveforms and CA-I , CB-I for the current waveforms.


First a few notes on hardware limitation issues. While the waveform generator in the ALM1000 system uses a 16 bit wide dynamic range DAC there is still significant wide band noise which can reduce the signal to noise ration when generating very small amplitude signals. The input signal level needed for the measurements in this lab activity is rather small because of the gain of the differential amplifier. If the waveform generator output were used directly the signal to noise ratio of its output is not high enough. The signal to noise ratio can be improved in this case by increasing the signal level and then placing an attenuator and filter ( single ended example shown in figure 1 ) between the generator outputs and the circuit inputs. The materials needed for this are:

1 - 47Ω resistor
1 - 1KΩ resistor
1 - 0.1uF capacitors (marked 104)

Figure 1 20:1 attenuator / filter example

An attenuator and filter like this will be used in all parts of this lab.


ADALM1000 hardware module
Solder-less breadboard and jumper wire kit
3 - 10KΩ resistors
1 - 47Ω resistor
2 - 1KΩ resistor
1 - 0.1uF capacitor (marked 104)
2 - Small signal NPN transistors (2N3904 or SSM2212 NPN matched pair)


Build the NPN differential pair test circuit shown in figure 2. The connections to the ALM1000 are as indicated in the green boxes. Q1 and Q2 should be selected from your available transistors with the best matching of VBE. The emitters of Q1 and Q2 share a common connection with one end of R3. The other end of R3is connected to ground (0V) and supplies the tail current. The base of Q1 is connected to the output of the first arbitrary waveform generator and the base of Q2 is connected to the output of the second arbitrary waveform generator trough the 20:1 attenuator network. The two collector load resistors R1 and R2 connect between the collectors respectively of Q1 and Q2 and the positive supply +5V. The two scope inputs CA-In and CB-In are used to measure the differential output as seen across the two 10KΩ load resistors.

Figure 2 Differential pair with tail resistor

Hardware Setup:

Both channels A and B should be set to the Split I/O mode with the Term set To 2.5V. Both generators should be configured for a 500 Hz sine wave, with Channel A phase set to 90 degrees and Channel B set to 270 degrees ( 180 degrees difference ). Set both channels with -11 mA Min value and +11 mA Max value. The resistor dividers will reduce the signal amplitude seen at the base of Q1with respect to the base Q2to slightly less than 200 mV. Channel CA-In and CB-In (Hi-Z inputs) should be connected to display the voltage at the collectors of Q1 and Q2.


Set to display all four traces. Turn on the display of the average voltage for the CA-V and CB-V traces. Set the channel A input offset equal to the average value. The displayed average should now be nearly zero. Do the same with channel B. Set the channel A and B position to 0 ( the CA-V and CB-V traces should now be centered on the grid. Turn on the CA-V - CB-V math trace, this will now display the differential output voltage.

By changing the value of R3, you should explore the effects of the level of the tail current on the gain of the circuit and the linear input range and the shape of the nonlinear (tanh) fall off in the gain as the circuit saturates. With minor additions to the basic circuit, such as emitter degeneration resistors, you should also explore techniques to extend and linearize the range of the input swing and the effects on circuit gain.

Using a current source as the tail current.

The use of a simple resistor as the tail current has limitations. You should explore ways to construct a current source to bias the diff pair. These could be made with a couple of additional transistors and resistors as in the stabilized current source from Activity 8.

Additional Materials:

2 - small signal NPN transistors ( Q3, Q4 2N3904 or SSM2212)

Figure 3 Diff pair with tail current source

Measuring Common Mode gain

Common mode rejection is a key aspect of the differential amplifier. CMR can be measured by connecting the base of both transistors Q1 and Q2 to the same input source. You should generate plots showing the differential output for both the resistively biased and current source biased differential pair as the common mode voltage is swept + and - 500mV around 2.5V.

Figure 4 Common Mode Gain configuration


Would you characterize this transistor amplifier as being inverting or non-inverting to outputs at each of the collector terminals with the base terminal of transistor Q1 being considered the plus (non-inverting) input? Explain your answer.

Describe what happens to each of the output voltages (CA-In and CB-In ) as the input voltage (CA-Out) increases, Decreases:

Suppose this differential-pair circuit was perfectly balanced. In this condition, how much voltage would be expected between the collector terminals of Q1 and Q2?

What is common-mode voltage, and how should a differential amplifier (ideally) respond to it?

Repeat the common mode gain measurements on the circuit shown in figure 3 with the tail current source. How has the result changed and why.

For Further Reading:

Return to ALM Lab Activity Table of Contents

university/courses/alm1k/alm-lab-12.1433383211.txt.gz · Last modified: 04 Jun 2015 04:00 by Doug Mercer