6.5. BJT DESIGN LIMITATIONS: NEED FOR BAND TAILORING 271
Intrinsic
Transistor
Cin b 0 re
Rbb
Figure 6.13: Basic BJT small-signal model for microwave power amplifiers.IfτB+τCis the dominant delay, then we may assumeτB+τC (ωT)−^1. Equation 6.5.27 can
then be written as
ω
ωT
>
1
β(6.5.28)
Since power amplifiers are rarely operated at frequencies a factor of 20 below the transit fre-
quency,β∼ 20 is in most instances adequate for these applications.
Microwave low noise amplifiers
The noise figure of low noise amplifiers is determined by the shot noise at the input. For BJTs
this is the shot noise of the base current 2 eIB, while for FETs it is that of the gate current 2 eIG,
wereIBis the base current of the BJT andIGis the reverse leakage current of the gate diode
of the FET. Since the forward bias base currentIB=IC/βis typically much larger than the
reverse bias currentIG, the minimum noise figure attainable at low frequencies(f<<fT)is
limited byβ, as shown in figure 6.14. Hence a highβ(typicallyβ> 100 ) is desirable for low
noise applications.
Logic applications
To explain the relevant issues in logic circuits, we will use a representative logic family, CML.
Let us assume one gate drivingngates, connected as shown in figure 6.15. Also assume that
nodeAis at a high. In the absence of base current, the difference between the voltage high and
the voltage lowΔVL=IoRL. However, in the presence of base current, since nodeCis high, a
currentnIBis sourced by the node. Hence the voltage at nodeCisVCC−nIBRL. This reduces
the logic difference to
ΔVL=−(VCC−IoRL)+(VCC−nIBRL)=(Io−nIB)RL=Io(
1 −
n
β)
RL (6.5.29)
To provide adequate noise margin, it is necessary that
n
β