8.8. SMALL AND LARGE SIGNAL ISSUES 411
8.8 SMALLANDLARGESIGNALISSUESANDFIGURESOFMERIT .....
It is important to understand the behavior of devices at higher frequencies both in small and
large signal operation. The former refers to applications such as low noise amplifiers in receivers
whereas the latter to applications such as power amplifiers used in transmitters.In this context
several figures of merit have been defined to characterize device performance. It is important to
recognize that frequencies of merit are in general a function of the application or equivalently
a function of the input and output networks that the device is connected to. In the following
sections we will study this in more detail and present in this introduction a short synopsis of the
treatment. The most important figure of merit is the current gain cut-off frequency,fτ,whichis
proportional to the inverse of the electron transit time across the device. The output termination
of the device whenfτis calculated is always an AC short circuit and hence reflects the device
behavior independent of the circuit. Thefτis the primary indicator of the average electron
velocity through the transistor and detailed analysis can extract electron velocity in regions of
the transistor. The power gain cutoff frequency of the devicefmaxis evaluated with the output
of the device presented with the complex conjugate of its output impedance to maximize power
transfer. This again is predominantly dependent on the device as the termination is determined
uniquely by the device characteristic. In other instances, like in large signal amplifiers driving
50 ohms, the load line is what determines the termination and hence another figure of merit,flsg
, the large signal figure of merit is used. In the discussion of the bipolar device high frequency
response we had to discuss minority carrier injection and removal. The FET is a majority carrier
device. The device performance is essentially controlled by carrier transit time effects. Thus
lithographic techniques defining the gate length and carrier mobility and velocity figure strongly
in device response.
8.8.1 Small-Signal Characteristics ......................
The equivalent circuit of a MESFET and the source of the various terms are shown in fig-
ure 8.39. A change of chargeΔQon the gate produces the changeΔQin the channel (assuming
charge neutrality). IfΔtis the time taken by the device to respond to this change, the change in
the current in the channel is
δID=δQ
Δt(8.8.1)
whereIDis the current flowing between the source and the drain. The timeΔtcan be inter-
preted as the average transit timettrfor the electrons to move through the device. The transistor
transconductance can be related to the transit time. The transistor intrinsic transconductance is