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. π π π π β 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. Ω = Ω = Ω = Ω = k R k R k R k R 10 1 10 60 4 3 2 1 179 1 5 5 2 1 = Ω = Ω = Ω = β k R k R k R E C C F C F C V V V V EE CC μ μ 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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