Student Zone-ADALM2000 Experiment: BJT Differential Pair

The purpose of this experiment is to study a simple differential Amplifier using NPN transistors. First, we need to do some explanations about hardware limitations. The waveform generator in the ADALM2000 system has a high output bandwidth, which replaces broadband noise. Due to the gain of the differential amplifier, the input signal level required for the measurement in this experiment is quite small. If the waveform generator is used directly for output, the signal-to-noise ratio of its output is not high enough.

Authors: Doug Mercer, Consulting Researcher, ADI; Antoniu Miclaus, System Application Engineer

Target

The purpose of this experiment is to study a simple differential amplifier using NPN transistors. First, we need to do some explanations about hardware limitations. The waveform generator in the ADALM2000 system has a high output bandwidth, which replaces broadband noise. Due to the gain of the differential amplifier, the input signal level required for the measurement in this experiment is quite small. If the waveform generator is used directly for output, the signal-to-noise ratio of its output is not high enough. By increasing the signal level, and then placing attenuators and filters (Figure 1) between the output of the waveform generator and the input of the circuit, the signal-to-noise ratio can be improved. The following materials are required for this experiment:

►Two 100 Ω resistors
►Two 1 kΩ resistors
►Two 0.1 μF Capacitors (marked as 104)

Student Zone-ADALM2000 Experiment: BJT Differential Pair
Figure 1.11:1 attenuator and filter

All parts of this experiment will use the attenuator and filter.

Differential pair with tail Resistor

Material

ADALM2000 Active Learning Module

►Solderless breadboard
►Jumper
►Two 10 kΩ resistors
►A 15 kΩ Resistor (connect a 10 kΩ resistor and a 4.7 kΩ resistor in series)
►Two small signal NPN transistors (2N3904 or SSM2212 NPN matched pair)

instruction

The breadboard connection is shown in Figure 3. Q1 and Q2 should be selected from the transistors available to you that have the best VBE matching. The emitters of Q1 and Q2 are connected to one end of R3. The other end of R3 is connected to Vn (-5V) to provide 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. Two collector load resistors R1 and R2 are respectively connected between the collectors of Q1 and Q2 and the positive power supply Vp (5V). Differential oscilloscope inputs (2+ and 2-) are used to measure the differential output ON two 10 kΩ load resistors.

Student Zone-ADALM2000 Experiment: BJT Differential Pair
Figure 2. Differential pair with tail resistor

Hardware setup

The first waveform generator is configured as a 200 Hz triangle wave with a peak-to-peak amplitude of 4 V and an offset of 0. The second waveform generator is configured as a 200 Hz triangle wave with a peak-to-peak amplitude of 4 V and an offset of 0 V, but the phase is 180°. The resistor divider reduces the signal amplitude at the base of Q1 and Q2 to slightly less than 200 mV. The 1+ pin in channel 1 of the oscilloscope is connected to the output of the first waveform generator W1, and the 1-pin is connected to the output of W2. Channel 2 is connected to the positions marked 2+ and 2- in the figure and set to 1 V per division.

Student Zone-ADALM2000 Experiment: BJT Differential Pair
Figure 3. Differential pair breadboard circuit with tail resistors

Procedure steps

Collect the following data: the x-axis is the output of the arbitrary waveform generator, and the y-axis is channel 2 of the oscilloscope using 2+ and 2-inputs. By changing the value of R3, explore the influence of the tail current level on the circuit gain (observe the slope of the straight line passing through the origin) and the influence on the Linear input range, and observe the shape of the non-linear gain when the circuit is saturated. Then add some small components to the basic circuit, such as emitter degradation resistance, to explore the technology of extending and linearizing the input swing range and its influence on the circuit gain.

Configure the oscilloscope to capture multiple cycles of the two signals measured. An example of the xy diagram is shown in FIG. 4.

Student Zone-ADALM2000 Experiment: BJT Differential Pair
Figure 4. Differential pair xy diagram with tail resistance

Current source used as tail current

Using a simple resistor as the tail current has limitations. The method of constructing a current source to bias the differential pair should be explored. This can be made up of several additional transistors and resistors, as shown in the previous ADALM2000 experiment “Stable Current Source”.

Additional materials

►Two small signal NPN transistors (Q3, Q4 = 2N3904 or SSM2212)

instruction

►Breadboard connection is shown in Figure 6.

Student Zone-ADALM2000 Experiment: BJT Differential Pair
Figure 5. Differential pair with tail current source

Hardware setup

The first waveform generator is configured as a 200 Hz triangle wave with a peak-to-peak amplitude of 4 V and an offset of 0. The second waveform generator should also be configured as a 200 Hz triangle wave, with a peak-to-peak amplitude of 4 V, an offset of 0 V, and a phase of 180°. The resistor divider reduces the signal amplitude at the base of Q1 and Q2 to slightly less than 200 mV. The 1+ pin of channel 1 of the oscilloscope is connected to the output of the first waveform generator W1, and the 1-pin is connected to the output of W2. Channel 2 is connected to the positions marked 2+ and 2- and is set to 1 V per division.

Student Zone-ADALM2000 Experiment: BJT Differential Pair
Figure 6. Differential pair breadboard circuit with tail current source

Program steps

Configure the oscilloscope to capture multiple cycles of the two signals measured. An example of the xy diagram is shown in FIG. 7.

Student Zone-ADALM2000 Experiment: BJT Differential Pair
Figure 7. Differential pair xy diagram with tail current source

Measuring Common-Mode Gain

Measuring common mode gain

Student Zone-ADALM2000 Experiment: BJT Differential Pair
Figure 8. Common mode gain configuration

Common mode rejection is a key aspect of differential amplifiers. CMR can be measured by connecting the bases of two transistors Q1 and Q2 to the same input source. The curve in Figure 10 shows the differential output of a resistor-biased differential pair and a current source-biased differential pair when the common-mode voltage of W1 is swept from +2.9 V to -4.5 V. The maximum positive swing on the input is limited to the point at which the base Voltage of the Transistor exceeds the collector voltage and the saturation voltage of the Transistor. This can be checked by observing that the collector voltage of the Transistor is single-ended with respect to ground (that is, the 2-oscilloscope input is grounded).

Hardware setup

The waveform generator is configured as a 100 Hz sine wave with a peak-to-peak amplitude of 8 V and an offset of 0. 1+ of channel 1 of the oscilloscope is connected to the output of the first waveform generator W1, and 1- is connected to the ground. Channel 2 is connected to the positions marked 2+ and 2- and is set to 1 V per division.

Student Zone-ADALM2000 Experiment: BJT Differential Pair
Figure 9. Common-mode gain breadboard circuit

Procedure steps

Configure the oscilloscope to capture multiple cycles of the two signals measured. The resulting waveform is shown in Figure 10.

Student Zone-ADALM2000 Experiment: BJT Differential Pair
Figure 10. Common mode gain waveform

problem:

For the circuit in Figure 8, if the base of transistor Q1 is regarded as the input, is the transistor amplifier inverted or in phase with respect to the outputs 2+ and 2-?

For the same circuit, explain what happens to each output voltage (2+ and 2-) when the input voltage (W1) increases. Please also explain what happens when the input voltage decreases.

You can find the answers to the questions on the student zone blog.

About the Author

Doug Mercer graduated from Rensselaer Polytechnic Institute (RPI) in 1977 with a bachelor’s degree in electrical engineering. Since joining ADI in 1977, he has directly or indirectly contributed more than 30 data converter products and holds 13 patents. He was appointed as an ADI researcher in 1995. In 2009, he transitioned from a full-time job and continued to serve as an ADI consultant as an honorary researcher, writing articles for the “Active Learning Program”. In 2016, he was appointed as the Resident Engineer of the RPI ECSE Department. Contact information:[email protected]