Low Frequency Response of BJT

Presenter: Aqsa AzizPhysics-V

When frequency is low enough, the coupling and bypass capacitors can no longer be considered as

shorts because their reactances are large enough to have significant effect. Also, when the frequency is

high enough, the internal transistor capacitances can no longer be considered as opens because their reactances become small enough to have significant effect on the

amplifier operation. This is called the frequency response of an amplifier.

In looking at the low frequency ac equivalent circuit of a capacitor coupled amplifier we can see there are three RC circuits which will limit low frequency response. The input at the base, the output at the collector, and the emitter.

A typical capacitively coupled common-emitter amplifier.

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

e

cmidv r

RA

')( 1

)( ' Ee

cmidv Rr

RA

1. The Input RC Circuit: For the BJT amplifier it is formed by C1 and the amplifier’s

input resistance.

in

Cin

inbase V

XR

RV

22

1

The frequency at which the gain is down by 3dB is called the lower critical frequency (fc).

inc

C RCf

X 1

1 2

1

12

1

CRf

inc

1)(2

1

CRRf

insc

The decrease in voltage gain with frequency is called the roll-off. A ten times change in frequency is called a decade. The attenuation measured in dB at each decade is the dB/decade. A plot of dB voltage gain vs. frequency is called a bode plot. Sometimes roll-off is expressed in dB/octave, which is a doubling or halving of the frequency.

In addition to reducing the voltage gain, the input RC circuit also causes an increasing phase shift through an amplifier as the frequency decreases.

in

C

RX 11tan

For midrange frequencies, Xc1 ≈ 0 Ω, so

o

inR0)0(tan

0tan 11

At the critical frequency, Xc1 = Rin, so

o

in

in

R

R45)1(tantan 11

A decade below the critical frequency, Xc1 = 10Rin, so

o

in

in

R

R3.84)10(tan

10tan 11

Phase angle versus frequency for the input RC circuit.

Input RC circuit causes the base voltage to lead the input voltage below midrange by an amount equal to the

circuit phase angle.

2. The Output RC Circuit: This circuit is formed by coupling capacitor C3, the

resistance at the collector and the load resistance RL.fc = 1/2(RC + RL)C3

Development of the equivalent low-frequency output RC circuit.

3. The Bypass RC circuit: It includes the bypass capacitor C2. For midrange frequencies it is

assumed that XC2 ≈ 0 Ω, effectively shorting the emitter to ground so that the amplifier gain is Rc/r’e. As frequency is reduced, XC2 increases. The impedance from emitter to ground increases, gain decreases. Av = Rc / (r’e + Re).

Development of the equivalent bypass RC circuit

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FET LOW FREQUENCY AMPLIFIER RESPONSE

A zero biased D-MOSFET amplifier with capacitive coupling on the input and

output .The midrange voltage gain of a zero biased amplifier is

THE INPUT RC CIRCUIT

the reactance of the input coupling capacitor increases as the frequency decreases

fcl(input)=

• The input resistance is • RIN =RG RIN(GATE)

RINGATE =

The phase angle in low frequency input RC circuit is θ= (Xc1/Rin)

Fcl(input)=

THE OUTPUT RC CIRCUIT

Fcl(output)=

The phase angle in the low frequency output RC circuit is

θ=

The phase angle is 45’ and approaches 90’ as the frequency approaches zero .however, starting at the

critical frequency, the phase angle decreases from 45’ and becomes very small as the frequency goes higher.