Page 219 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

FIGURE 13-12 Q1 collector output current for various amplitude inputs at
the base of Q1. The.-\axis is time, and the >axis is the output current.
In Figure 13-12, the input levels at the base of Q1 are peak sinusoidal input levels
of 26 mV, 52 mV, 104 mV, and 416 mV, respectively. At 26 mV peak, the output

current still looks low in distortion, with about 2 percent third-order harmonic
distortion. But as the amplitude input level is increased from 52 mV to 416 mV
peak, we see that the output current increases in distortion. Distortion is seen at
104 mV, with a soft clipping waveform changing to hard clipping resembling a
square wave at 416 mV at the input.

As a result of this distortion, the fundamental frequency signal current at the
collector of Q1 will compress as the input signal is increased. So, although the
distortion is symmetric compared with the distortion of a single-ended transistor
amplifier, as shown in Figure 13-10, there is also a drop in transconductance in a
differential-pair transistor amplifier as the input signal is increased.
Table 13-4 shows the approximate' drop-off in large-signal transconductance as a

function on input voltage across the bases of the differential amplifier. Note that
the large-signal transconductance is output-signal current of the fundamental
frequency divided by the input-signal voltage of the fundamental frequency.