Page 209 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

From Figures 13-6 through 13-8 we see that when the input of a bipolar transistor

amplifier is overdriven with a large signal, there is a lowering of effective
transconductance pertaining to signals with the fundamental frequency. So how
does this lowering of the effective transconductance of the transistor owing to
overloading affect other characteristics such as input resistance in a bipolar
transistor amplifier?
In a grounded base amplifier, the AC input small signal resistance at a quiescent

collector current is just 11 VBE/I1I E , where I1I E is the change in emitter current. But
we know that in a transistor with current gain ~. > 10, for practical purposes, I1IE 5
I1Ic , the input resistance is I1VBE/I1Ic , which is just the reciprocal of gml where gm =

I1Idl1VBE = I /O.026 volt. So the small-signal input resistance of a common-base
cQ
amplifier at a quiescent collector current, ICQ is l/g m = 1/(IcQ/O.026 volt).
Also note for a common-emitter amplifier with the emitter AC grounded the input
resistance is 11 VBE/ I1IB , where 111B is the change in base current. But J31B = lc,

where ~ is the current gain, and I1IB = I1Id~. Because of ~, the small-signal input
resistance of a common-emitter amplifier with emitter AC grounded is J3(I1VBE/I1I c )
= ~/gm.

Now that the input resistances of common-base and common-emitter amplifiers
have been established, let's take a look at the effect of input resistances for small
and large signals at the input where the input resistances pertain to the
fundamental frequency signal (i.e./ harmonics are ignored). Effectively, the
large-signal resistance is the small-signal resistance multiplied by a factor of gm/Gm.

Intuitively, the reason why the input resistance across the base emitter junction (or
emitter-ta-base junction) increases as the input signal is increased. With large
signals at the input, the input currents are no longer sinusoidal but instead are
distorted in such a manner that the fundamental frequency input current decreases
while the input currents of the harmonics increase. For example, when the sinusoid

input voltage signal is > 26 mV peak, the input current has a periodic narrowed
pulse waveform. The frequency spectrum of this periodic narrowed pulse waveform
consists of signals of the fundamental frequency and at least the first few
harmonics that are almost equal in amplitude. For example, a 1 mA peak-ta-peak
periodic narrowed pulse waveform (see Figure 13-8 with 52 mV peak AC signal)
does not have a 1 mA peak to peak fundamental frequency sinusoidal component.
Instead, the fundamental frequency signal component of the narrowed pulse is

smaller « 1 mA peak-to-peak) because the narrowed pulse contains harmonics as
well. And because of the smaller fundamental signal amplitude current at the input,
the input resistance increases as the input voltage signal is increased to cause
distortion in the form of a narrowed pulsed input current waveform.

As can be seen in Table 13-3, driving a common-base amplifier with a sine wave of
100 mV peak (i.e., 200 mV peak to peak) actually will increase the input resistance