0195136047.pdf

410 TRANSISTOR AMPLIFIERS

Magnitude Av 0

Magnitude of gain

Av0

2

Midband

W3dB

Phase angle

of gain

Phase

0

Deterioration

due to

coupling capacitors

and bypass

capacitor

0 ωL ωH ω

Deterioration

due to capacitances

internal to the

transistor

Bandwidth (passband)

Figure 8.5.1Typical frequency response of a voltage amplifier.

ac while isolating them for dc are limited in their low-frequency response. The high-frequency

deterioration of the voltage gain is due to the effect of capacitances that are internal to the transistor.

Figure 8.5.1 shows a typical frequency response of a voltage amplifier. While most often the

magnitude of the gain is discussed (because it defines the frequency range of useful gain), the

phase response becomes important for transient calculations. Themidbandregion is the range of

frequencies where gain is nearly constant. Amplifiers are normally considered to operate in this

useful midband region. Thebandwidthof an amplifier is customarily defined as the band between

two frequencies, denoted byωHandωL, which corresponds to the gain falling to 3 dB below the

midband constant gain. Thus,

W3dB=ωH−ωL (8.5.1)

which is shown in Figure 8.5.1. When bothωLandωH have significant values, the amplifier

is known as a bandpass amplifier. A narrow-bandpass amplifier is one in whichW3dBis small

relative to the center frequency of the midband region. In a low-pass unit (dc amplifier) there is

no low 3-dB frequency.

The general problems of analyzing any given amplifier to determineωLorωH, and of

designing amplifiers with specific values ofωLandωH, are extremely complex and well beyond

the scope of this text. Specific amplifier cases can, however, be considered in order to gain a sense

of what is involved in determining the frequency response. Toward that end let us consider the CS

JFET amplifier shown in Figure 8.4.1(a). The small-signal ac equivalent circuit for low frequencies

nearωLis depicted in Figure 8.5.2(a), that for high frequencies nearωHin Figure 8.5.2(b).

Assumingroto be large for convenience, the gain of the low-frequency circuit of Figure

8.5.2(a) can be found to be

Av=

vL

vs

=

Av 0 (j ω)^2 (ωZ+jω)

(ωL 1 +j ω)(ωL 2 +j ω)(ωL 3 +jω)

(8.5.2)