Bipolar transistors

What does a differential pair, with the emitters of two transistors connected to a common current source, do?

  1. AAmplifies the sum of the two input voltages
  2. BAmplifies the difference between the two input voltages
  3. CRectifies the input voltage
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Answer: B. Amplifies the difference between the two input voltages

In a differential pair, the current from the common current source is divided between the two transistors according to the difference between the two input voltages. If both inputs change by the same amount, the division does not change, so only the difference is amplified. It is used as the input stage of op-amps and similar circuits.

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More Bipolar transistors questions

  1. What is the approximate transconductance gm of a bipolar transistor operating at a collector current of 1 mA? (Room temperature, thermal voltage VT ≈ 26 mV)
  2. What is the main use of a photocoupler, which combines an LED and a phototransistor in one package?
  3. What is the main reason for using a current-feedback bias circuit with an emitter resistor in a transistor amplifier?
  4. In an amplifier, what is the DC voltage and current state of the transistor with no input signal called?
  5. Which distortion tends to occur in a class B push-pull amplifier in which NPN and PNP transistors handle the positive and negative half-cycles?