This version (23 Aug 2019 13:07) was approved by Antoniu Miclaus.The Previously approved version (12 Jul 2019 13:38) is available.Diff

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Activity: BJT Differential pair


To investigate the simple differential amplifier using NPN transistors. First a few notes on hardware limitation issues. The waveform generator in the ADALM2000 system has a high output bandwidth and with that high bandwidth comes wide band noise. 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 by increasing the signal level and then placing an attenuator and filter ( figure 1 ) between the generator outputs and the circuit inputs. The materials needed for this are:

2 - 100Ω resistors
2 - 1KΩ resistors
2 - 0.1uF capacitors (marked 104)

Figure 1 11:1 attenuator and filter

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


ADALM2000 Active Learning Module
Solder-less breadboard
Jumper wires
2 - 10KΩ resistors
1 - 15KΩ Resistor (use a 10KΩ in series with a 4.7KΩ)
2 - Small signal NPN transistors (2N3904 or SSM2212 NPN matched pair)


The connections to the Lab hardware are as indicated in figure 2. 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 the Vn (-5V) 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. The two collector load resistors R1 and R2 connect between the collectors respectively of Q1 and Q2 and the positive supply Vp ( +5V ). The differential scope input 2 +/- is used to measure the differential output as seen across the two 10KΩ load resistors.

Figure 2 Differential pair with tail resistor

Hardware Setup:

Figure 3 Differential pair with tail resistor Breadboard Circuit

The first waveform generator should be configured for a 200 Hz Triangle wave with 4 volts amplitude peak-to-peak and 0 offset. The second generator should be configured also for a 200 Hz Triangle wave with 4 volts amplitude peak-to-peak and 0 volts offset but with 180 degree phase. The resistor dividers will reduce the signal amplitude seen at the bases of Q1and Q2to slightly less than 200 mV. Channel one of the scope should be connected with 1+ to the output of the first generator, W1 and 1- connected to W2. Channel 2 should be connected to display 2+ and 2- and set to 1V per division.


The following data should be taken: the X axis is the output of the arbitrary waveform generator and the Y axis is scope channel 2 using both the 2+ and 2- inputs. By changing the value of R3, you should explore the effects of the level of the tail current on the gain of the circuit (as seen in the slope of the line as it passed through the origin) 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.

Figure 4 Differential pair with tail resistor XY plot

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 section 5. above.

Additional Materials:

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

Figure 5 Diff pair with tail current source

Hardware Setup:

Figure 6 Differential pair with tail current source Breadboard Circuit


Same procedure as for the tail resistor.

Figure 7 Differential pair with tail current source XY plot

Measuring Common Mode gain

Figure 8 Common Mode Gain configuration

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. The plot below shows the differential output for both the resistively biased and current source biased differential pair as the common mode voltage from W1 is swept from +2.9V to -4.5V around ground. The maximum positive swing on the input is limited to the point where the base voltage of the transistors exceed the collector voltage and the transistors saturate. This can be checked by observing the collector voltage of the transistors single ended with respect to ground (i.e. grounding the 2- scope input.)

Hardware Setup:

Figure 8 Common Mode Gain Breadboard Circuit


Figure 9 Common Mode Gain Waveform


Would you characterize this transistor amplifier as being inverting or non-inverting to outputs 2+ and 2- with the base terminal of transistor Q1 being considered the input? Explain your answer.

Describe what happens to each of the output voltages (2+ and 2- ) as the input voltage (W1) 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.


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