Electronic Circuits Laboratory EE462G Lab #8. BJT Common Emitter Amplifier

Transcription

1 lectronic ircuits Laboratory 46G Lab #8 JT ommon mitter Amplifier

2 npn ipolar Junction Transistor JT in a common-emitter configuration ase ollector V _ n p n V _ mitter For most applications the JT is operated in the active region where: V 0.6V and V > V

3 npn JT Operation JT in a common-emitter configuration in active region (V > V ~.6V): V _ n n i (-) V _ The pn junction for V is forward biased and current i flows according to the Shockley equation: i V = I S exp 1 VT where V T.6 mv and I S ranges from 10-1 to lectrons from the emitter flow into the base and are pulled into the depletion region of the reversed biased collector-base junction.

4 npn JT haracteristics JT Transfer haracteristics in active region with β = I / I = 100: 1 x x 10-4 Amps (ase urrent) Amps (ollector urrent) I = 4 microamps I = 3 microamps I = microamps I = 1 microamp Volts (V) Volts (V)

5 D (iasing) Model I V quations for the D operating point, assume I <<<< I : V I 1 V = V 1 I V = I V I I = βi I I = I Key esult (Load-line quation): I I = V V β 1 β 1 β β How sensitive is I c to changes in β?

6 D (iasing) quivalent Model Apply Thévenin model to base terminal: 1 V V V = V 1 = 1 1 V I = Load-Line quation V V ( 1 β ) ( 1 β ) V I

7 Load Line Analysis: Amps (ase urrent) V ( 1 β ) x Volts (V) V V β β 1 Amps (ollector urrent) 5 x If β changes, I changes I = 1 microamp1 I = 4 microamps I = 3 microamps I = microamps Volts (V) V Qualitatively describe what happens in both curves when β increases(or decreases).

8 Large-Signal (D) Model I 1 V I I βi V I V V I V I I

9 JT Amplifier Once the D operating point is set, the A input and output are coupled to the amplifier with capacitors so as not to perturb the operating point. V s in 1 out v s L

10 JT Amp Small-Signal Model onsider the capacitors as short circuits for the small signal A and open circuit for D to obtain the model below. The resistor r o accounts for the small slope of the I-V characteristics in the forward-active region (often assumed to be infinite). The resistance r π is found from linearizing the nonlinear base-emitter characteristic, which is an exponential diode curve. v s s v in - i r π βi r o v out - L = 1 1

11 JT Amp Small-Signal Model Determine the voltage gain. How would the emitter resistor affect the gain if it was not bypassed? v s s v in - i r π βi r o v out - L replaces the short here!

12 Taylor Series ecall that a function can be expressed as a polynomial through a Taylor Series expansion: f ( x) = ( x a) f ( a) 1! df ( x) dx x= a ( x a)! f ( x) x= a where a is a point about which the function is expanded. Note that if a represents a quiescent point for a voltage, then the reciprocal of the coefficient first linear represents the small signal impedance. d dx

13 SPI xample The amplifier circuit can be constructed in SPI using the JT npn (Q) part from the menu. The edit simulation model option can then be used to set the ideal forward beta The input can be set to a sinusoid at desired frequency and amplitude for a transient analysis. SPI can also do a Fourier analysis to observed effects of clipping and distortion. There should be no harmonic energy for perfect amplification.

14 SPI xample xample circuit with meters to monitor input and output: 1 10K 3 1K 3 1u V u Q1 beta= 100 IVm V 0 5k u 6 1K IVm

15 SPI xample Graphic output for.03 V sine wave input (IVM1) at 10 khz. Output is shown for meter IVM. What would the gain of this amplifier be at 10 khz?

16 SPI xample Fourier analysis of output. Frequency magnitude plot for output at meter IVM. bjtexama-fourier-0-graph Frequency (Hz) k k k k m 0.0 freq norm_mag_v D(freq) D(norm_mag_v8)

17 SPI xample Graphic output for.08 V sine wave input (IVM1) at 10 khz. Output is show for meter IVM. What would the gain of this amplifier be at 10 khz?

18 SPI xample Fourier analysis of output. Frequency magnitude plot for output at meter IVM. bjtexama-fourier--graph Frequency (Hz) k k k k m 0.0 freq norm_mag_v D(freq) D(norm_mag_v8) -.653Meg

19 SPI xample Graphic output for 1. V sine wave input (IVM1) at 10 khz. Output is show for meter IVM. What would the gain of this amplifier be at 10 khz?

20 SPI xample Fourier analysis of output. Frequency magnitude plot for output at meter IVM. bjtexama-fourier-3-graph Frequency (Hz) k k k k m 0.0 freq norm_mag_v D(freq) D(norm_mag_v8)

21 JT ircuit Parameters How can β be found experimentally using the curve tracer? How can the input and output resistances be determined experimentally? How can voltage gain be determined experimentally?