Lab Report 5

Lab Report 5 - Scott Smith ENEE306-0104 21 November 2005...

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Unformatted text preview: Scott Smith ENEE306-0104 21 November 2005 Lab Report Lab 5: Differential Amplifiers and Op-Amp Basics 5.1 Introduction Differential amplifiers are used throughout electronics. Op-amps are built from differential amplifiers as we will design in this lab. This lab will investigate the significance and implications of feedback in the differential amplifier circuit. 5.2 Differential Amplifiers Differential amplifiers are useful because of their large gain and the ability to reject noise. Figure 1 5.2.1 DC Bias Q 3 acts as a constant current source for the differential pair. We find the DC bias of the circuit in figure 1 by treating the AC sources as shorts to ground. I am assuming that Q 1 and Q 2 are equivalent and emitter current is equal to collector current in all transistors. This means that the emitter current is split evenly between the transistors Q 1 and Q 2 . EE E E I I I = + 2 1 (1) 2 2 1 EE E E I I I = = (2) 1 1 1 C C CC C R I V V- = (3) 2 2 2 C C CC C R I V V- = (4) R 1 R 2 v in 1 R C 1 C 1 R E Q 3 Q 1 Q 2 v in 2 R C 2 C 2-V EE V CC Smith 0104 V V V E E 7 . 2 1- = = (5) To ensure maximum voltage swing, choose R C such that 2 CC C V V 2245 . 5.2.2 Differential Pair Small Signal Voltage Gain The output can be taken at either Q 1 collector or Q 2 collector. The voltage gain is different depending on which output is used. 2 1 2 12 C m in o v R g v v A = = (6) 2 1 1 11 C m in o v R g v v A- = = (7) When both signal sources are applied and the output is taken at Q 2 collector, the total voltage gain is 2 2 1 2 C m in in o dm R g v v v A =- = (8) Input and Output Resistance We can use what was learned in Lab 2 to find the input resistance of a common emitter transistor. However we must modify the formula slightly. The emitter resistance in the Lab 2 equation will be translated as effective emitter resistance, so it will be the resistance seen by looking into Q 2 emitter. r r r g r R m in 2 1 2 1 2 1 = + = + = (9) The output resistance is the same as in Lab 2 for a common emitter transistor. 2 C o R R = (10) 2 Smith 0104 5.2.3 Experiment Figure 2 Step1: Use the circuit in figure 2 with the following values. = = = = k R k R k R k R 10 1 10 60 4 3 2 1 179 1 5 5 2 1 = = = = k R k R k R E C C F C F C V V V V EE CC 1 . 1 . 10 10 2 1 = = = = Step 2: Measure the DC values at the collectors of Q 1 and Q 2 . How well do the measurements agree with the theoretical values?...
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Lab Report 5 - Scott Smith ENEE306-0104 21 November 2005...

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