Circuit Description; General - HP 200AB Operating And Servicing Manual

3-1 GENERAL
The circuit of the Model 200AB consists of an oscil-
lator section, a push-pull amplifier section, and a
conventional power supply.
The oscillator section consists of a 6SJ7 (V1) anda
6K6 (V2) and is basically a two stage resistance
coupled amplifier. Two feedback loops are used
around this amplifier, positive feedback to set up
oscillations, a negative feedback to reduce distortion
E Positive Feedback
Ou
LAMP
Rit
Negative
Feedback
Figure 3-1.
SE
Sect.1 Page 1
SECTION Iil
CIRCUIT DESCRIPTION
and keep the amplitude of oscillation constant. The
positive feedback network contains fixed resistance
values and a variable capacitor (see Figure 3-1) is
proportioned such that Rj C1 = Rg Cg. The oscil-
lator output voltage is applied to this network and
the oscillator input voltage is derived from it. Since
the input and output signals of a two stage amplifier
are in phase, oscillations will occur when the phase
shift between the voltage applied to the network and
the voltage at the grid of the first stage is zero.
The zero phase shift point is also the point of mini-
R1I6
RI & R2 Correspond to range
resistors RI-R8
Cl & C2 Correspond to ci! thru c4
Simplified Oscillator Circuit