BJT and FET frequency response

We will now investigate the frequency effects introduced by the larger capacitive elements of the network at low frequencies and the smaller capacitive elements of the active device at high frequencies .

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FREQUENCY RESPONSE

BJT AND FET

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We will now investigate the frequency effects introduced by the larger capacitive elements of the network at low frequencies and the smaller capacitive elements of the active device at high frequencies

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Low, High & Mid Frequency Range

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Typical Frequency Response

The band frequencies define a level where the gain or quantity of interest will be 70.7% of its maximum value

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• Phase plot

• Decibel plot

• Normalized plot

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 Defining the Low Cutoff Frequency

In the low-frequency region of the single-stage BJT or FET amplifier, it is the RC combinations formed by the network capacitors CC, CE, and Cs and the network resistive parameters that determine the cutoff

frequencies

LOW FREQUENCY ANALYSIS- BODE PLOT

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Voltage-Divider Bias Config.

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shown by the change in gain from fL/2 to fL.

• For a 10:1 change in frequency, equivalent to one decade, there is a 20-dB change in the ratio, as

demonstrated between the frequencies of fL/10 and fL.

• The piecewise linear plot of the asymptotes and associated breakpoints is called a Bode plot of the magnitude versus frequency.

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Cs : Cc:

=> the capacitors Cs, CC , and CE

will determine the low-frequency

response.

fL= max(fLs , fLc , fLE)

Cs :

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Impact of RS

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FroydWess - Online Notes

Bode Plot

A Bode plot indicates the

frequency response of an

amplifier.

The horizontal scale

indicates the frequency (in

Hz) and the vertical scale

indicates the gain (in dB).

Figure 11.4 Gain versus frequency: (a)

RC-coupled amplifiers; (b) transformerRC-coupled

amplifiers; (c) direct-coupled amplifiers

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Miller input capacitance

- In the high-frequency region, the

capacitive elements of importance are

the interelectrode (between-terminals)

capacitances internal to the active

device and the wiring capacitance

between leads of the network

- For any inverting amplifier, the input

capacitance will be increased by a

Miller effect capacitance sensitive to

the gain of the amplifier and the

interelectrode (parasitic) capacitance

between the input and output terminals

of the active device

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• A positive value for A v would result in a negative capacitance (for Av > 1)

• For noninverting amplifiers such as the common-base and emitter-follower configurations,

the Miller effect capacitance is not a contributing concern for high-frequency applications

• The Miller effect will also increase the level of output capacitance, which must also be

considered when the high-frequency cutoff is determined

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Network Parameters

 At high frequencies, the various parasitic capacitances (Cbe, Cbc, Cce) of the

transistor are included with the wiring capacitances (CWi, CWo).

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FET Amplifier

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Cutoff Frequencies:

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

The bandwidth is defined

by the lower and upper cutoff frequencies.

Cutoff – any frequency at which the gain has

dropped by 3 dB.

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BJT Amplifier Low-Frequency Response:

At low frequencies, coupling

capacitor (C S , C C ) and

bypass capacitor (C E )

reactances affect the circuit

impedances.

Figure 11.16 Loaded BJT amplifier with capacitors that

affect the low-frequency response.

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BJT Amplifier Low-Frequency Response

The Bode plot indicates

that each capacitor may

have a different cutoff

frequency.

It is the device that has

the highest lower cutoff

frequency (f L ) that

dominates the overall

frequency response of the

amplifier.

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Roll-Off of Gain in the Bode Plot

The Bode plot not only indicates the cutoff frequencies of the various capacitors it also indicates the amount of attenuation (loss in gain) at these

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Coupling Capacitor (C G )

The cutoff frequency due to

CG can be calculated with

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Coupling Capacitor (C C )

CC can be calculated with

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Bypass Capacitor (C S )

CS can be calculated with

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FET Amplifier Low-Frequency Response

The Bode plot indicates that

each capacitor may have a

different cutoff frequency.

The capacitor that has the

highest lower cutoff

frequency (f L ) is closest to

the actual cutoff frequency of

the amplifier.

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BJT Amplifier High-Frequency Response

Capacitances that affect the

high-frequency response are

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Input Network (f Hi ) High-Frequency Cutoff

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Output Network (f Ho ) High-Frequency Cutoff

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hfe (or β) Variation

β 2πβ mid r e

(C be + C bc )

The hfe parameter (or β) of

a transistor varies with frequency

f ≅ 1

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BJT Amplifier Frequency Response

Note the highest lower cutoff frequency (fL) and

the lowest upper cutoff

frequency (fH) are closest to the actual response

of the amplifier.

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FET Amplifier High-Frequency Response

Capacitances that affect the

high-frequency response are

CS Figure 11.52 Capacitive elements that affect the high

frequency response of a JFET amplifier.

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Input Network (f Hi ) High-Frequency Cutoff

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Output Network (f Ho ) High-Frequency

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Multistage Frequency Effects

Each stage will have its own frequency response, but the output of one stage will be affected by capacitances in the subsequent stage This is especially so when determining the high frequency response For example, the output

Miller Capacitance (C Mi ) of the next stage.

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Multistage Amplifier Frequency

Response

Once the cutoff frequencies have been determined for each stage (taking into account the shared capacitances), they can be plotted.

Note the highest lower cutoff frequency (fL) and the lowest upper cutoff

frequency (fH) are closest to the actual response of the amplifier.

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Square Wave Testing

In order to determine the frequency

response of an amplifier by

experimentation, you must apply a wide

range of frequencies to the amplifier.

One way to accomplish this is to apply a

square wave A square wave consists of

multiple frequencies (by Fourier

analysis: it consists of odd harmonics).

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Square Wave Response Waveforms

If the output of the

amplifier is not a perfect

square wave then the

amplifier is ‘cutting’ off

certain frequency

components of the square

wave.

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