FREQUENCY RESPONSE BJT AMPLIFIER
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 ------ 2 pF ~ 50 pF C = Cbc ------ 0.1 pF ~ 5 pF
Basic data sheet for the 2N2222 bipolar transistor Cob = Cbc Cib = Cbe Output capacitance Input capacitance
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.
Miller equivalent circuit
V1 V2
Miller Capacitance Effect V1 V2 C V1 V2
High-frequency hybrid- model ro C V gmV C - + C = Cbe C = Cbc
High-frequency hybrid- model with Miller effect ro CMi gmV C CMo A : midband gain
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
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
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
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