Lab Experiment #2 Differential Amplifiers. Group Members

Transcription

1 Lab Experiment #2 Differential Amplifiers Group Members Student 1 Student 2 Student 3 Student Name Surname First Name Student ID # Pre-Lab Mark (out of 30) Lab Demo and performance (out of 70) Total Lab Mark TA Signature:

2 Objective: The objective of this lab is to construct a crude differential amplifier from discrete components. Prelab Work 1) A current source is to be implemented using a current mirror. The circuit is shown in Figure 1. Assuming the transistors to be matched each with a β of 200, what value of R is needed so that the current I = 2 ma? Figure 1 Current Mirror 2) Suppose that we replace the transistors in Figure 1 with two matched transistors with β = 100. For the same R found in 1), what is the new value of I? What happens to I if we change transistors again so that β = 500? 3) For the differential amplifier shown in Figure 2, find gm, rπ, re, Rid and Ad.

3 10 V 10 V 3 k 3 k + v d - 2 ma -10 V Figure 2. amplifier / Amplificateur différentiel 4) Repeat 3) for a differential amplifier with 20 Ω resistors connected to the each emitter. 5) For the circuit shown in fig. 2, find the single ended common mode gain if the current source has a 100 kω input resistance. 6) Suppose that the load resistance on Q2 is Rc + Rc, where Rc = 0.05Rc, what is the common mode gain for the differential output? What is the CMRR (in db)?

4 Procedure / Procédure Part 1 Current Mirror 1) 2 2N3904 transistors are needed for this part of the experiment. β is measured for the transistors. Try to select two transistors whose β values are very close. 2) Build the circuit shown in fig. 3. What current I, do we expect if the transistors are assumed to be perfectly matched? Vary the resistance Rx from 1 kω in steps of 1 kω and measure the voltage across Rx. From this determine I. Stop increasing Rx when I has decreased to one-half its original value. Graph I as a function of Rx. Use the graphs sheet given on the following page. 3) At what value of Rx does I start decreasing and why does this happen? I starts decreasing at R x = 10 V 6.8 kω R x I -10 V -10 V Fig. 3 Circuit to implement in Part 1

5 I R x

6 Part 2 Differential Amplifier 1) 2 new 2N3904 transistors are needed. Once again students must select 2 which have very similar values for β. 2) Build the circuit in fig. 4 by removing the resistance Rx from the circuit in fig. 3 and building the differential amplifier part on top of it. The resistance RB is needed to supply the transistor with a DC base current. The capacitors are needed to force the DC current into the base. Measure the DC voltages in the transistors collectors and bases. What is the linear range of the output signals; what is the voltage offset of your amplifier? Write those values in the table below. 10 V Rs RB RC RC RB C1 Vd Vs - + B Q1 + Vo - Q2 C1 A Rid RK R B = 2.2 MΩ R S = 100 KΩ -10 V Fig. 4 Differential Amplifier Circuit V C1 V C2 V B1 V B2 Linear Range of the output signal Voltage Offset

7 3) Using a 100 mv amplitude (200 mvp-p) 5 khz sine wave input, measure the vd. Since vd/vs= (Rid RB)/(Rs+(Rid RB)). Use this to find Rid. Rid = 4) Measure the amplitude of the output. Find the differential gains Ad = vo/vd as well as Ads = vo/vs. Make sure that you correctly measure the differential output vo1 vo2. Assuming the transistors to be matched, what differential gain is expected? Vo Ad A ds Expected Voltage Gain 5) Disconnect the base of the second transistor from ground and connect it to point B in fig. 4. This is common mode. Observe the output vo and measure its amplitude. If the output appears as a clipped or distorted sine wave, you may be required to reduce the amplitude of vs. Find the common mode gain Acm = vo/vcm. Compute the CMRR in db (20log(Ad/Acm)). v o A cm CMRR in db