LAB 5: DIFFERENTIAL AMPLIFIER

Title:

Differential Amplifier

Objective:

The differential amplifier is the fundamental building block of virtually all integrated circuits. The common parameters desired are the differential gain, common-mode gain, input offset voltage, common-mode rejection ratio, and common-mode input voltage range. This exercise examines differential amplifier implemented with BJTs.

Equipment and components:

4 BJT transistors (2N3904); 1 Multi-turn potentiometer (1 kW); and resistors (values determined in lab); 15 V and -15 V power supply; function generator.

Note:

Analyze the circuit first, which includes hand calculations or computer simulation. Compare your experiment measurement with your analysis predictions. Implement the Widlar current source from the previous lab to get the desired current output.

 

Procedure

Build the differential amplifier shown below. Implement your previous Widlar current source to get the input current (current through R1) of 10 mA and the collector current of Q4 of 1 mA.

  1. Ground v+ and v- and adjust the multi-turn potentiometer so that VC1 equal VC2. Measure IC1, IC2, and VE. Calculate the differential voltage gain using the equation below and the input offset voltage.

    2. Ad = ( VC1 - VC2 ) / VD = - gmRC (where VD = v+ - v-).

    Also, measure the bias current IB flowing into the base of Q1 and Q2. Calculate the offset current |IB1 - IB2|.

  2. Let v+ = 2 mV and v- = -2 mV. Measure VC1, VC2, and VE. Calculate the differential voltage gain and the single-ended voltage gain at Q2. Repeat this step for v+ = |v-| = 4 mV, 6 mV, 8 mV, and 10 mV. (VC1, VC2, and VD are the small signal changes from their quiescent zero input values from (a)). From these data, plot the differential voltage gain (VC2 - VC1)/VD vs. VD and the single-ended differential voltage gain VC2/VD vs. VD. Indicate the range of VD where the gains are constant.
  3. Let v+ = v- = vCM. Vary vCM from + 15 V to - 15 V. Find the range of vCM where Q1 and Q2 are in the active mode.
  4. Let vCM = sin (2p f t) with f = 1 kHz. Measure the common-mode voltage gain vc2/vCM.
  5. Calculate the common-mode rejection ratio (CMRR) in dB.