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SERVICE DATA
lntroduction
The pre-amplifier consists of an AC coupled HF-
amplifier and a DC coupled LF-amplifier the outputs
of which are combined by a cross-over filter. The
latter forms a high-pass filter for the HF-amplifier
and a low-pass filter for the DC-amplifier. The HF-
amp'lifier has a bandwidth from 300 Hz to 100 MHz;
the Dc-amplifier
has a bandwidth from DC to
4 MHz. The -3dB
frequency of the cross-over filter
lies at 100 kHz. In this way a split-band amplifier is
obtained with a bandwidth from DC to 100 MHz.
The advantages of the split-band amplifier are:
fl. lower noise
H.F. transistors suffer fro,m a serious | /t noise
at frequencies lower than approx. 100 kHz. L.F.
transistors cause a serious noise at higher fre-
quencies. Moreover, they do no,t meet the rise-
time requirements. In the split-band amplifier the
best properties of the transistors are used, each
in their own frequency range.
b. beffer DC biasing
In a split-band amplifier it is possible to adjust
the transistons to their optimum collector-cuÍïent
i.e. for H.F. transistors to approx. l0 ffiA, for
L.F. transisto,rs to approx. I mA.
c. No necessity for constant dissipation networks
As the time-constant of the thermal processes in
tl
the H.F. transistors amounts to several milli-
seconds and the H.F. amplifier is followed by a
100 kIJz high-pass filter, there is no necessity
to provide the H.F. amplifier with constant dissi-
pation networks.
d. Better decoupling of the supply-voltages
As RC coup,ling is used, it is only necess ary to
have one co,mmon power supply. The supply per
stage is sufficiently decoupled without impairing
the L.F. square-wave response. To avoid undue
interstage coupling the emitter resistors are con-
nected to earth.
It is possible to switch o'ff the Ac
amplifier with
switch SKI so that a DC amplifier remains with a
bandwidth of 0...100 kHz, without HF noise.
At the 4 smallest deflection coefficients
5 my / cm
to 0,5 mY / cm - the amplification factor of the split-
band amplifier is increased by decreasing the negative
feedback. A convenient deflection can be selected
with switch SK4.
For the larger values, attenuator networks are insert-
ed between the input terminal and the pre-amplifier.
The DC-amplifier is provided with a control circuit
(chopper amplifier) which counteracts DC drift.
The final amplifier, contrary to the pre-amplifier, is
entirely push-pull connected. The output of the final
amplifier is routed to, bo,th the y-amplifier
and
the trigger amplifier of the basic o'scilloscope.
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