1. Lecture V Low Frequency Response of BJT Amplifiers
DMT 231/3 Electronic II
Lecture V
Low Frequency Response of
BJT Amplifiers

2. Effect of Coupling Capacitors
mid-band frequencies:
coupling & bypass capacitors
 shorts to ac
low frequencies:
capacitive reactance affect the gain & phase shift of signals
 must be taken into account

3. FREQUENCY RESPONSE : the change in gain or phase shift over a specified range of input signal frequencies

4. Effect of Coupling Capacitors
 capacitive reactance varies inversely with frequency

5. Effect of Coupling Capacitors: Example
Figure 1

6. Effect of Coupling Capacitors: Example
At low freq (audio freq: 10 Hz)
 less voltage gain (then they have at higher freq): Figure 1
More signal voltage is dropped across C1 & C3 (higher reactances : reduce voltage gain)
Reactance of C2 becomes significant & the emitter is no longer at ac ground.

7. Nonzero reactance of the bypass capacitor in parallel with RE creates an emitter impedance, (Ze), which reduces the voltage gain.
Figure 2

8. Effect of Internal Transistor Capacitances
Internal transistor capacitances reduce amplifier gain & introduce phase shift as the signal frequency increases Cbe: base-emitter junction capacitance Cbc: base-collector junction capacitance
Figure 3

9. AC equivalent circuit for a BJT amplifier showing effects of the internal capacitances Cbe and Cbc.
Figure 4

10. General case of Miller input and output capacitances. C represents Cbc
Miller’s Theorem
General case of Miller input and output capacitances. C represents Cbc
Figure 5

11. Amplifier ac equivalent circuits showing internal capacitances and effective Miller capacitances.
Figure 6

12. Decibel
Logarithmic measurement of the ratio of one power to another OR one voltage to another
Ap(dB) = 10 log Ap
Ap = Pout / Pin
Av(dB) = 20 log Av
Av = Vout / Vin

13. O dB Reference
Reference gain (no matter what its actual value is)
 used as a reference with which compare other values of gain
Midrange gain
Maximum gain occurs for the range of freq between the upper & lower critical freq
Normalized
Midrange voltage gain is assigned a value of 1 or 0 dB.

14. Normalized voltage gain versus frequency curve.
Figure 7

15. Power Measurement in dBm
Critical Frequency
Also known as cutoff frequency OR corner frequency
Frequency at which the output drops to one-half of its midrange value.
Corresponds to a 3 dB reduction in the power gain:
Ap(dB) = 10 log (0.5) = - 3 dB
Power Measurement in dBm
dBm : unit for measuring power levels referenced to 1 mW

16. A capacitively coupled amplifier
Low Frequency Amplifier Response
A capacitively coupled amplifier
Figure 8

17. The low-frequency ac equivalent circuit of the amplifier consists of three high-pass RC circuits.
Figure 9

18. Input RC circuit formed by the input coupling capacitor and the amplifier’s input resistance.
Figure 10

19. Input RC Circuit
The base voltage for the input RC circuit:
when

20. Lower critical frequency
Condition where the gain is down 3 dB: overall gain is 3 dB less than at midrange freq  a.k.a lower cutoff freq, lower corner freq, or lower break freq.
Taking into account the input source resistance

21. dB voltage gain versus frequency for the input RC circuit
Bode Plot
Decade: ten times change in frequency Bode Plot: a plot of dB voltage gain versus frequency on semilog graph paper
Figure 11
dB voltage gain versus frequency for the input RC circuit

22. Phase angle versus frequency for the input RC circuit.
Phase angle in an input RC circuit
For midrange frequencies
At critical frequency
At a decade below the critical frequency
Figure 12

23. The input RC circuit causes the base voltage to lead the input voltage below midrange by an amount equal to the circuit phase angle, .
Figure 13

24. Development of the equivalent low-frequency output RC circuit.
Phase Shift in Output RC Circuit
Figure 14

25. At low frequencies, XC2 in parallel with RE creates an impedance that reduces the voltage gain.
Figure 15

26. Development of the equivalent bypass RC circuit.
Figure 16

27. Total Low Frequency Response of Amplifier
Composite Bode plot of a BJT amplifier response for three low-frequency RC circuits with different critical frequencies. Total response is shown by the blue curve.
Figure 17

28. Composite Bode plot of an amplifier response where all RC circuits have the same fc. (Blue is ideal; red is actual.)
Figure 18

29. High Frequency Response of
BJT Amplifiers

30. Coupling & bypass capacitors: effective shorts Internal capacitance: significant ONLY at high frequencies
Capacitively coupled amplifier and its high-frequency equivalent circuit.
Figure 19

31. High-frequency equivalent circuit after applying Miller’s theorem.
Figure 20

32. Development of the equivalent high-frequency input RC circuit.
Figure 21

33. Example: Derive the equivalent high-frequency input RC circuit for the BJT Amplifier in Figure 22

34. Development of the equivalent high-frequency output RC circuit.
Figure 24

35. High-frequency Bode plots.
Figure 26

36. A BJT amplifier and its generalized ideal response curve (Bode plot).
Figure 27