Philips PM 3342 Manual page 17

23
Vertical am~li Fier
1. Pre-amplifier channel A (Fig. 21)
The incoming signal is fed directly or via a capacitor
depending on the position of switch SK3 (AC-DC), to
a 10-step attenuator, which reduces the signal amplitu-
de by afactor 1, 2, 5, 10 etc. up to 1000, as selected with
switch SK4 (V/cm).
Cathode-follower B201 provides an impedance trans-
formation to drive the following transistor amplifier.
As the cathode-follower is subject to some DC-drift, a
second cathode follower B202 is symmetrically con
netted to it to minimise drift. Furthermore, provision
is made to balance the amplifier by varying the grid
potential of B202 at R321 "COARSE DC-BALANCE"
and R2 "DC-BAL".
When properly adjusted, no signal will be present
across R3 (CONTINUOUS CONTROL) if the grid
circuit of B201 is connected to earth with switch SK1
("CHECK ZERO").
Clamping diodes GR202 and GR203 prevent the tran-
sistor amplifier (TS201-TS202) driving signal from
becoming intolerably high in either the positive or the
negative direction. Protecting diode GR201, which is
normally forward biased, p~ events the maximum
(negative) grid-to-cathode voltage from being exceeded,
because if such a signal is present, protecting diode
GR201 will be reversely biased. The cathode voltage
will then be determined by the —150 Volts supply and
the voltage-drop across R206 due to the cathode cur-
rent. To complete the temperature-compensation, the
cathode circuit of B202 is likewise provided with diode
GR206 of the same type. Diode GR204 prevents high
voltages from being applied to TS202.
The second stage is formed by emitter-followers TS201
and TS202. They offer a low impedance signal source
for the next stage and for the trigger-amplifier, which
receives its signals via R232 and R237. From conti-
nuous control R3, the signal is routed to the third stage,
TS203-TS204.
This stage can be regarded as an asymmetrically driven
long tailed-pair amplifier.
The output of this stage consists of two equal, but
opposite, signals. GAIN control R4 influences the
negative feedback and consequently the amplification
factor. R235 and C211 provide a correction to obtain
optimum step-response.
Shift controls Y-SHIFT (R1-R1') and the polarity
switch SK2 are inserted between the third and fourth
stage. The connections from the third stage to SK2 and
from SK2 to the fourth stage are effected by means of a
300 Ohm ribbon line, which is terminated by R249 and
the series-connection of R254-R256. The power is
supplied at the junction of R254-R256. In this way, the
total dynamic resistance at the collectors of TS203 and
TS204 amounts to 75 Ohms.
The fourth stage consists of two cascode amplifiers
(TS206-TS208 and TS207-TS209) which are push-pull
connected. Transistors TS206 and TS207 act as current
sources to the low-ohmic inputs of TS208 and TS209
respectively. Thus hardly any signal voltage will be
present at the emmitters of TS208 and TS209, so it is
possible to carry out the electronic switching at this
point.
If the voltage at R508 becomes more positive than the
emitter voltage, switching diodes GR209 and GR211
are blocked, thus allowing normal operation of the
cascode-amplifiers. If the voltage at R508 is decreased
below the potential at the bases of TS208 and TS209,
the transistors are fully blocked and diodes GR208 and
GR212 likewise. So no signals will pass and channel A
is cut off.
In the emitter circuit of TS206 and TS207, R263 and
C217 provide low frequency compensation; C218
improves the step-function response.
2. Pre-amplifier channel B (Fig. 21)
This pre-amplifier is identical to that of channel A. The
switching voltage of channel B however, is counter-
phased. Thus the diodes GR309-GR311 are conductive
when GR209-GR211 are blocked and vice versa .As a
result, the signal of channel A is displayed when
channel B is blocked and vice versa.
3. Output amplifier (Fig. 21)
The driving signals of the output amplifier are devel-
oped across R402 and 8406. Emitter-followers TS401-
TS402 offer these signals with a low source impedance
to cascode-amplifiers TS403-TS406 and TS404-TS407
The amplified signals are finally routed to the output
stage consisting of emitter followers TS408-TS409
which make them available to the vertical amplifier of
the basic oscilloscope via terminals 1 and 17 of BU-Y.
When the MODE-switch SKS is in position ADDED,
diodes GR209-GR211-GR309-GR311 are blocked
simultaneously so that both channels are switched in
and a combined signal is applied to the output ampli-
fier. To maintain the same voltage-level at R404, R401
is connected in parallel.
The two cascode-amplifiers are connected in push-pull
arrangement with a high common resistance at the
emitter side of TS403-TS404 thus forming one differen-
tial amplifier. C422, C401 and R413 improve the step-
functionresponse.