1. FREQUENCY RESPONSE
BJT AMPLIFIER

2. HIGH FREQUENCY
The gain falls off at high frequency end due to the internal capacitances of the transistor.
Transistors exhibit charge-storage phenomena that limit the speed and frequency of their operation.
Small capacitances exist between the base and collector and between the base and emitter. These effect the frequency characteristics of the circuit.
C = Cbe pF ~ 50 pF
C = Cbc pF ~ 5 pF

3. Basic data sheet for the 2N2222 bipolar transistor
Cob = Cbc
Cib = Cbe
 Output capacitance
 Input capacitance

4. Miller’s Theorem
This theorem simplifies the analysis of feedback amplifiers.
The theorem states that if an impedance is connected between the input side and the output side of a voltage amplifier, this impedance can be replaced by two equivalent impedances, i.e. one connected across the input and the other connected across the output terminals.

5. Miller equivalent circuit

6. V1 V2

7. Miller Capacitance EffectV1 V2 C V1 V2

8. High-frequency hybrid- modelro C V
gmV C - +
C = Cbe
C = Cbc

9. High-frequency hybrid- model with Miller effectro
CMi
gmV C
CMo
A : midband gain

10. High-frequency in CE amplifier
22 k
VCC = 10V
4.7 k RS C1
10 F C2 C3
600 
470 
2.2 k R1 RC RE R2 vS RL
Given :
 = 125, Cbe = 20 pF, Cbc = 2.4 pF, VA = 70V, VBE(on) = 0.7V vO
Determine :
i-Upper cutoff frequencies
ii- Dominant upper cutoff frequency

11. High-frequency hybrid- model with Miller effect for CE amplifier
R1||R2 RS
RC||RL vs vo r ro C
CMi
CMo
gmV
 midband gain
 Miller’s equivalent capacitor at the input
 Miller’s equivalent capacitor at the output

12.  Thevenin’s equivalent resistance at the input
 Thevenin’s equivalent resistance at the output
 total input capacitance
 total output capacitance
 upper cutoff frequency introduced by input capacitance
 upper cutoff frequency introduced by output capacitance

13. How to determine the dominant frequency
The lowest of the two values of upper cutoff frequencies is the dominant frequency.
Therefore, the upper cutoff frequency of this amplifier is