How oscilloscopes work (2): circuitry I93
TR2 should have both a high collector dissipation rating and a
low Ccb. Note that if TR2 is changed for another type with twice
the dissipation rating, enabling the standing current to be
doubled and
R E
halved, the bandwidth would be increased even
though the
Ccb of
the more powerful transistor were twice that of
the original one. This is because
Ccb 2
generally constitutes less
than 50 per cent of Ct, which will therefore have increased by a
much smaller factor than two.
Inductive peaking
A bandwidth greater than the above f-3 dB can be obtained by the
use of inductive peaking circuits to offset the effect of Ct. Note
that Ct includes the collector capacitor of the plate-driving
transistor, the capacitance of the connecting lead to the plate, and
the effective plate capacitance. The last is generally listed by the
c.r.t, manufacturer
as Cyl_all,
meaning the capacitance of one Y
plate to everything else
except
the other Y plate, and Cyl_y2
meaning the capacitance between the Y plates. The effective plate
capacitance is Cpe = Cyl-aU + Cy~_y2 if only one plate is driven, or
Cpe = Cy~_an + 2Cy~_y2 if, as is usually the case, the two Y plates are
driven in antiphase.
Figure 10.4(a) shows a deflection-amplifier output stage using
shunt peaking. If we define Q such that Q =
L/RLXCt,
then if L is
chosen such that Q = 0.25 the pulse response of the stage will
show no overshoot, while for Q = 0.414 there will be 3.1 per cent
overshoot. However, the risetime will be 71 per cent and 59 per
cent respectively of that of the same stage without the inductive
peaking. By using a capacitance C = 0.22Ct in parallel with a value
of peaking inductance L = 0.35RL2Ct, the risetime falls to 56.5 per
cent of the uncompensated value and the overshoot is only 1 per
cent.
The above are examples of 'two-terminal' compensation
networks; improved performance at the expense of increased
complexity can be obtained by splitting Ct into its component
parts. Ccb
and Cpe are compensated separately; the capacitance of
the plate connection lead can be included with either of these
two to help make up the relative values of capacitance shown in
194
Oscilloscopes
Note
-ci
15
not
o
seporote
I
(b)
component but
the
I
Ccbol
TR2
over:hoot
(10%
shown)
(C)
10
%
Figure I0.4jbj.
With
this four-terminal peaking circuit,
the
risetime
is
only
40
per cent
ol
that
of
the amplifier without
compensation, and ovedioot is
less
than
1
per
cent. The
improveinent
in
Frequency
response
is
inuch
less
marked than
the reduction
iii
riselime, although
if
dillerent
L
and
C
values are
chosen
a
circuit can
hc produccd
with
a
~req~reiicy-response
level
up
to
2.4 times the
-3dR
point
of
tlic
uncompensated amplifier.
However.
lhis
is
01
Iiniited use
in
aii
oscilloscope
as
it
shows
a
marked dcgrce of overshoot
on
last
pitlses.
Overshoot
is
illustrated in Figure 10.4(cj.
The
wholr
sribjt:r~
of
pvaking is covrrcd
sitccinct.ly
in
Chapt.er
9
of
Elcctrants
cirrd
RudL?
Em!Inwt!iLy
by
F.
E.
Terman,
McGraw-Hill,
4th
edition,
1955,
where
an
exlensive
list
of
lurther
rel'erencrs
can
he
foiind.
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
! R 318 § v
9 r sqot:) ? ~ T T /
t o '~ I
J ~- u I E421 1 ~01 ~ I C303 4~v ~ ~ I
, ~ k14 .... t r---wr-,r o'.,, ~ rl~ ..... .n T o ,, ,
I /
U "~ . i L.~,,~. ~
....... ~ I
I | i -~ 3v r ~ : ~z jv ;a)~] /m.~.-~,~v., 't , .,,..,., z --,.., ~.
I e----~-'~4~ ' 1uzr )~C~Oe ~.~06( ~ I
I / ~ .... I ,c 3o, I ,7
~-~/ k,~
/ I I
§
v ~,to 3 ,5, 144 5 B I
/
'~
,
t __ I E421 ~-.~I I I I 0 "n I I I ||C))O J
Y' ...... " ..... V,'; "~ '
, I ~'~ ~" I i
~- "" __J i_
,
,
,~,~~ ,~
l~,,,.L
~o o,~
..~, , ~, ,o ~ ,
9 ~ 4~. 9 _r-----~ l | qf'~ J J J l
!
4.
1,50
tr,q o~tl~t to ~b~
.............
u
j~- I 1,,v
Y ~l.~ft
lror~ VQ20 ~
. ~
I
~2~, I
'4
to
c~o~s,s ~ i ~c., .'ho',',,'.
r !
L
I
Figure 10.5 Y-deflection amplifier designed by the author for use with c.r.t, type 3BP1
How oscilloscopes work (2): circuitry 197
,/ x ................ / %
1 /
/ /
Figure 10.6 Output of slew-rate limited amplifier for three increasing input
amplitudes of an ideal squarewave
change is determined by Ct and the magnitude of the tail current.
Thus, in contrast to inductive peaking, with emitter compensa-
tion fast squarewaves are reproduced more faithfully when they
are displayed at small amplitude than when displayed at full
screen height. Likewise, the -3 dB bandwidth is greater for small
deflections than large; this explains the growing practice of
quoting bandwidths at half-screen deflection, which might
possibly be reasonable in the case of dual-trace instruments, but
is really not fair in a single-channel scope.
As mentioned earlier, the Y deflection stage of an oscilloscope
is run at the highest possible standing current, limited by
considerations of device dissipation, in order to achieve the
widest possible bandwidth. This has an unfortunate side-effect in
an unsophisticated circuit such as that shown in Figure 10.5.
Imagine that an a.c. waveform with a standing positive d.c. level
is to be displayed, and that therefore the trace has been set at the
bottom of the graticule in readiness. Consequently, TR305 and
303 are conducting much more heavily than TR306 and 304.
Therefore (assuming thermal equilibrium has been reached) the
base-emitter voltage (Vbe) of each of the former pair of transistors
will be less than that of the latter pair, since they will be hotter
and Vbe has a temperature coefficient of around-2 mV/~ Now,
when the signal is applied, the trace will be nearer the top of the
graticule, so that the dissipation in TR306 and 304 will exceed
that in TR305 and 303. Over the next few seconds as the latter
two transistors cool and the former heat up, the base-emitter
voltages will change accordingly. This will give rise to a spurious
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