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Differential Amplifiers Differential amps take two input signals and amplify the differences (“good” signal) while rejecting their common levels (“noise”)

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Presentation on theme: "Differential Amplifiers Differential amps take two input signals and amplify the differences (“good” signal) while rejecting their common levels (“noise”)"— Presentation transcript:

1 Differential Amplifiers Differential amps take two input signals and amplify the differences (“good” signal) while rejecting their common levels (“noise”) Normal-mode input: differential changes in the input signals Common-mode input: both inputs change levels together A good differential amp has a high common-mode rejection ratio (CMRR) of about 10 6 (120 dB) –Ratio of response for normal-mode signal to response for common-mode signal of the same amplitude Differential amps help us to understand operational amplifiers (coming in Lab 8)

2 Differential Amplifiers in Electrocardiography (Analog Electronics for Scientific Application, D. Barnaal, Waveland Press, 1989)

3 Differential Amplifier Construction (The Art of Electronics, Horowitz and Hill, 2 nd Ed.) (“+” or “non-inverting” input) (“–” or “inverting” input) (“single-ended” output)

4 Differential Amplifier Construction “Long-tailed” pair configuration: (The Art of Electronics, Horowitz and Hill, 2 nd Ed.)

5 Differential Amplifier of Lab 6–1 + input – input output (Student Manual for The Art of Electronics, Hayes and Horowitz, 2 nd Ed.) Q1Q1 Q2Q2

6 Differential Amplifier Performance (Student Manual for The Art of Electronics, Hayes and Horowitz, 2 nd Ed.)

7 Differential Amplifier Performance (Student Manual for The Art of Electronics, Hayes and Horowitz, 2 nd Ed.)

8 Differential Amplifier Performance: Improving CMRR (The Art of Electronics, Horowitz and Hill, 2 nd Ed.) (Lab 6–1)

9 Single-Ended Input Differential Amplifier + input output (not inverted) (The Art of Electronics, Horowitz and Hill, 2 nd Ed.) (Lab 6–1)

10 Example Problem 2.13 Solution details given in class. (The Art of Electronics, Horowitz and Hill, 2 nd Ed.) Verify that and. Then design a differential amplifier to your own specifications.

11 Bootstrapping “Standard” emitter follower biasing scheme: (The Art of Electronics, Horowitz and Hill, 2 nd Ed.)

12 Bootstrapping “Bootstrapping” increases Z in at signal frequencies without disturbing the DC bias: (The Art of Electronics, Horowitz and Hill, 2 nd Ed.) (Lab 6–2)

13 Bootstrap Design Want Thévenin resistance of bootstrap network at DC to be same as Thévenin resistance of bias voltage divider in original circuit (10k) Choose R 3 = 4.7k Then R 3 + R 1  R 2 = 10k  R 1  R 2 = 5.3k ≈ 5k Choose R 1 / R 2 = 1 (same as original circuit) –Solve for R 1 and R 2 from the above  R 1 = R 2 = 10k Choose f 3dB and calculate C 2 or choose C 2 and calculate f 3dB  using C 2 = 10  F, f 3dB = 3.2 Hz –We do the latter since we don’t know choice of f 3dB Similarly, choose C 1 and calculate f 3dB,in –For C 1 = 0.1  F, f 3dB,in = 16.9 Hz

14 Transistor Junction and Circuit Capacitance (The Art of Electronics, Horowitz and Hill, 2 nd Ed.)

15 Miller Effect Consider the following amplifier with voltage gain –G, with a capacitor connected between input and output: –The effective input capacitance becomes C eff = C(1 + G) According to the Miller model, the equivalent input circuit is: C eff (Graphics from

16 Miller Effect Source impedance (R source ) and C eff form a low-pass filter with an f 3dB smaller than without Miller Effect (Student Manual for The Art of Electronics, Hayes and Horowitz, 2 nd Ed.) (C Miller = C eff )

17 Defeating Miller Effect Reduce R source (R source = 0 eliminates Miller Effect) Arrange things so that base and collector of any one transistor do not head in opposite directions at the same time (Student Manual for The Art of Electronics, Hayes and Horowitz, 2 nd Ed.)

18 Defeating Miller Effect Cascode circuit (Student Manual for The Art of Electronics, Hayes and Horowitz, 2 nd Ed.) (Lab 6–3)

19 Beating Miler Effect Single-ended input differential amplifier (Student Manual for The Art of Electronics, Hayes and Horowitz, 2 nd Ed.)

20 Darlington Connection (Student Manual for The Art of Electronics, Hayes and Horowitz, 2 nd Ed.) VCVC V E = 0 V V B ≈ 1.2 V ≈ 0.6 V (Lab 6–4) ICIC IBIB

21 Superbeta Transistor Superbeta transistor used in Lab 6–5 (The Art of Electronics, Horowitz and Hill, 2 nd Ed.) (Lab 6–5)


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