Transistor Frequency Response and Miller Effect

Transistor Frequency Response

Transistor Frequency Response

•Low frequency response

•Miller effect capacitance

•High frequency response

After studying the material in this chapter, you should be able to:

Define bandwidth, cutoff frequency, and geometric center frequency, and identify each on a frequency-response curve.

Calculate any two of the following values, given the other two: fC1, fC2, geometric center frequency (f0), or bandwidth.

Compare and contrast the Bode plot with the frequency-response curve.

Perform the low-frequency-response analysis of an FET amplifier.

Perform the high-frequency-response analysis of an FET amplifier.

General Frequency Considerations Frequency response of an amplifier refers to the

frequency range in which the amplifier will operate with negligible effects from capacitors and capacitance in devices.

This range of frequencies can be called the mid-range.

At frequencies above and below the midrange, capacitance and any inductance will affect the gain of the amplifier.

General Frequency Considerations At low frequencies the coupling and bypass

capacitors will lower the gain.

At high frequencies stray capacitances associated with the active device will lower the gain.

Also cascading amplifiers will limit the gain at high and low frequencies.

General Frequency Considerations

A frequency-response curve is a graphical representation of the relationship between amplifier gain and operating frequency.

A generic frequency response curve is shown in Figure next slide. This particular curve illustrates the relationship between power gain and frequency.


A generic frequency-response curve


The bandwidth of the circuit is found as the difference between the cutoff frequencies. By formula:

BW = fC2 – fC1

General Frequency Considerations The geometric center frequency (f0) of an

amplifier is the geometric average of the cutoff frequencies, found as

Power gain is maximum when an amplifier is operated at its geometric center frequency. As frequency varies above (or below) f0, the power gain decreases slightly. By the time one (or the other) cutoff frequency is reached, power gain has dropped to half its midband value

Robert BoylestadDigital Electronics

Copyright ©2002 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458

All rights reserved.

The mid-range frequency of an amplifier is called the Bandwidth of the amplifier.

The Bandwidth is defined by the Lower and Upper Cutoff frequencies.

Cutoff: frequency at which the gain has dropped by:

0.5 power0.707 voltage-3dB

Cutoff Frequencies

Bode Plot

Low Frequency Response – FET Amplifier


CCGG

Cutoff frequency


CCCC

Cutoff frequency


CCSS

Cutoff frequency

ExampleExample

SolutionSolution

Miller Effect Capacitance

High-Frequency Response –BJT Amplifier

Network Parameters

• At increasing frequencies, the reactance Xc will decrease in magnitude, resulting in a shorting effect across the output and a decrease in gain.

• The derivation leading to the corner frequency for this Rc configuration follows along similar lines to that encountered for the low-frequency region.

• The most significant difference is in the general form of AV, appearing below:


For the input networkFor the input network


For the output networkFor the output network

High-Frequency Response – FET Amplifier

Capacitances that will affect the high-frequency response:• Cgs, Cgd, Cds – junction capacitances• Cwi, Cwo – wiring capacitances• CG, CC – coupling capacitors• CS – bypass capacitor


For the input networkFor the input network


For the output networkFor the output network

Transistor Frequency Response and Miller Effect

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