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Modifications to the Basic Transistor Model

Modifications to the Basic Transistor Model. So far, we’ve assumed a simple model for transistors: V BE = constant = 0.6 V I E ≈ I C = b I B But this simple picture breaks down when we try to get greedy:

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Modifications to the Basic Transistor Model

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  1. Modifications to the Basic Transistor Model • So far, we’ve assumed a simple model for transistors: • VBE = constant = 0.6 V • IE≈ IC = bIB • But this simple picture breaks down when we try to get greedy: • The basic model predicts G   as RE 0, but this is not the case • “Electronic Justice” prevails! (The Art of Electronics, Horowitz and Hill, 2nd Ed.)

  2. Ebers-Moll Equation • Gain is limited because VBEvaries with IC according to the Ebers-Moll equation: • IS = saturation current (depends exponentially on temperature) • VT = kT / q = 25.3 mV at room temp. • k = Boltzmann’s constant, T = temperature in Kelvin, q = charge of the electron • Since IC >> IS(on)  IC≈ IS exp(VBE / VT) • VBEincreases by≈ 60 mV for every decade increase in IC or by ≈ 18 mV for every doubling of IC (Lab 5–1,5–4) (Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)

  3. Implications of Ebers-Moll Equation • There is an intrinsic resistance associated with the emitter (re) • Pretend it is a resistor in series with the emitter (Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.) (Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)

  4. Implications of Ebers-Moll Equation • Temperature effects • VBEdecreases by≈ 2.1 mV per °C, holding IC constant • ICincreases at about 9% per °C, holding VBE constant • From massaging the equation a little (Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)

  5. Complication: Early Effect • IC grows as the voltage across the transistor (VCE) grows, caused by changing effective base width • If IC is assumed fixed, this effect looks like an effect on VBE: (a ≈ 0.0001) • Another interpretation of the Early Effect is that there is a variation in ICwhile VBE is fixed • Thus transistors are not perfect current sources! (Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)

  6. Emitter-Follower Revisited • re prevents the output impedance from getting too small Vout Vin RE RL (The Art of Electronics, Horowitz and Hill, 2nd Ed.) (Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)

  7. Common-Emitter Amplifier Revisited • re caps the gain of the grounded-emitter amplifier • Assume IC(quiescent) = 1 mA. Then re= 25 W and G ≈ –200, but G is not constant since re is not constant (IC not constant) (Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)

  8. Common-Emitter Amplifier Revisited (Lab 5–2) • This change in gain (from the changing re) results in a “barn-roof” distortion of the output waveform: (Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)

  9. Common-Emitter Amplifier Revisited • Fix: add an emitter resistor much larger than re • RE reduces output signal error at the cost of gain • Electronic justice prevails again! • Typically don’t use a grounded common-emitter amplifier (Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)

  10. Negative Feedback • RE also provides beneficial “negative feedback”: • IC begins to grow with increased temperature • VE rises, as a result of increased IC (Ohm’s law) • Rise in VE lowers VBE since VB is fixed, and so IC is reduced according to Ebers-Moll equation (like closing the transistor “valve”)  better temperature stability (Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)

  11. Common-Emitter Amplifier Revisited (Lab 5–2) • For temperature stability and high gain, bypass RE with a capacitor (remember Lab 3–6): • RE “disappears” at high frequency as ZC 0 • But this circuit still distorts the output signal Helps temp. instability problem   Helps improve gain (Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)

  12. More Negative Feedback (Lab 5–5) • Another example of (DC) negative feedback: • The negative feedback makes the quiescent DC level of the collector more stable • However the feedback is quite small (< 0.1%) at signal frequencies, so signal gain is essentially undisturbed = RC VC (quiescent) ≈ 7 V VC R1 = 0.6 V 0 V R2 = (Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)

  13. Matched Biasing Transistor • Matched transistor pairs are used to stabilize base voltage for the required collector current • Ensures automatic temperature compensation (Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)

  14. Current Mirrors (Lab 5–3) • The concept of matched base-emitter biasing is useful in making current mirrors VE = 15 V VB = 14.4 V VC = 14.4 V ≈Iprogram Iprogram (Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)

  15. Temperature Effects on Current Mirrors • However Iout does not match Iprogram very well when the temperatures of the two transistors become unequal (Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)

  16. Temperature Effects on Current Mirrors • You can beat the effects of temperature variations by using matched transistors on a monolithic IC array (i.e. made on the same chunk of silicon) • Both transistors stay at the same temperature (Lab 5–3) (Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)

  17. Early Effect on Current Mirrors • Another problem: Iout and Iprogram will differ to the extent that VCE differs across each transistor (consequence of Early Effect) VE = 15 V VB = 14.4 V VC = 14.4 V (Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)

  18. Defeating Early Effect in Current Mirrors • Fix: add an emitter resistor that provides negative feedback to fight the growth of Q2’s IC • The cost is a somewhat decreased compliance range of Q2 +15 V Q2 Q1 VE VB VC (Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)

  19. Defeating Early Effect in Current Mirrors (Lab 5–3) • A better fix: use the Wilson Mirror circuit • Clamps VCE for both Q1and Q2 so a relative change in VCE does not occur between them +15 V 14.4 V 14.4 V 13.8 V (Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)

  20. Push-Pull Emitter Follower • Suppose you wanted to drive a low-impedance (8 W) speaker using an emitter-follower transistor: • A rather large base current of at least 50 mA is needed for Q2in order to meet the power requirements of the speaker (9 Vrms across 8 W) • More power is consumed in the circuit than is delivered to the speaker (The Art of Electronics, Horowitz and Hill, 2nd Ed.)

  21. Push-Pull Emitter Follower • A push-pull follower does the same job but with much smaller power dissipation • Average power dissipated in each transistor is about 2 W, considerably less than the 10 W delivered to the load Speaker behaves like a load resistor to ground: Rspeaker (The Art of Electronics, Horowitz and Hill, 2nd Ed.)

  22. Push-Pull Emitter Follower (Lab 5–6) • An undesirable consequence of this circuit is “crossover distortion” when –0.6 V  Vin  +0.6 V • See text for a solution to this problem Q1conducting Q2conducting (The Art of Electronics, Horowitz and Hill, 2nd Ed.)

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