How oscilloscopes work (2): circuitry 195
Emitter compensation
With the inductive peaking schemes described above, the
improvement in risetime over an uncompensated amplifier is
independent of the amplitude of the displayed trace, and is
limited to a factor of about 2.5:1 using a four-terminal compensa-
tion network. The trend recently has been to abandon inductive
peaking of deflection-amplifier output stages in favour of emitter
compensation.
This scheme is exemplified in Figure 10.5, which shows the
circuit of a Y amplifier designed by the author for minimum
risetime when using a 3BP1, an insensitive and very outmoded
design of c.r.t., but cheap and readily available. Here, the gain of
the output amplifier output stage at d.c. and over most of its
frequency range is determined by R326, but at higher frequencies
C309, 310 tend to bypass R326, resulting in a gain that rises with
frequency, compensating for the loading effect of Ct. In fact, the
gain of the amplifier transistors is also beginning to fall, with the
result that it is not a simple RC load circuit that we are trying to
compensate. Consequently, additional components R325, C311
and R308, C314 are included to ensure the smooth roll-off of the
frequency response necessary for the faithful reproduction of
pulse waveforms.
This type of circuit makes use of the fact that a deflection
amplifier is always designed to be able to overscan the available
screen display area by up to 100 per cent or more, so that the spot
can be deflected way beyond the top or bottom of the graticule.
When a very fast rising edge is applied to the Y amplifier, the
long-tailed pair TR305, 306 will be overdriven, as their emitters
are tied together by C309, 3 I0. The result is that all the available
tail current (set by R333; TR307, 308 serve only to introduce the
Y shift voltage) is momentarily diverted through, say, TR305
while TR306 is cut off. The load capacitance Ct at each collector is
therefore charged at the maximum possible rate set by the
available tail current. As Ct charges, so do the emitter-compensa-
tion capacitors C309 and C310, resulting in the steady-state
deflection being reached with minimal overshoot.
This deflection amplifier is said to be 'slew-rate limited' (Figure
10.6), as the maximum speed at which the Y-plate voltage can