MM0474
transient should cause the output of Q615 to go negative. Power amplifier Q601
then drives the bridge through terminal E13 and bridge current meter M301.
(4) Solid state amplifiers using bipolar transistors in the input stage
exhibit two types of non-ideal behavior which must be compensated for, if the
best accuracy is required. The first of these is an equivalent voltage offset
at the input terminals, E4 and E5, which is due to imperfect matching of the
two halves of Q609.
This offset is cancelled by unbalancing the collector
circuits of Q609 with R625 and either R618 or R619, depending on whether the
instrument is being used with a barretter or a thermistor.
These two
adjustments are made as described in para. 4 by nulling the voltage between
terminals E4 and E5.
These voltages are available at the front panel
terminals.
(5) The second type of non-ideal behavior is the bias current required
by the two halves of Q609. These currents must be drawn through R304 and R305
if not otherwise provided. To the extent that the bias currents flow through
the bridge, they will produce an error in the substituted DC power measurement.
Therefore, the currents drawn by Q609 at terminals 1 and 2 are individually
compensated by constant current generators. One of these is composed of Q602
and Q603, and the other of Q606 and Q607, with the associated resistors. These
two sources introduce currents into terminals 1 and 2, which are independent of
the DC voltage between those terminals and signal common. The adjustment of
these current sources is also described in para. 4 and consists of nulling the
currents which pass through terminals E4 and E5.
(6) The self-balancing bridge is essentially a servo-mechanism and as
such, the AC characteristics of the balancing amplifier are important. It is
necessary to provide AC feedback loops within the amplifier to compensate the
servo-mechanism for the characteristics of the load which it drives, in this
case, the bridge itself, which includes the bolometer. Since the bolometer may
be either a thermistor, which is relatively slow, or a barretter, which has a
shorter time constant, the AC compensation networks must be changed according
to the setting of the barretter-thermistor switch.
These networks are
associated with high impedance portions of the circuit, and in order to avoid
the effects of variation of stray lead capacitances, the networks are located
on the B600 board and are switched by field effect transistors controlled by
the barretter-thermistor switch.
There are three such networks on the B600
board. The two networks switched by Q604 and Q616 are in use in the barretter
position. Alternately, the network switched by Q605 is used in the thermistor
position.
g. Reference Generator Circuit Description.
(1) The reference generator has three basic modes of operation as
outlined in para. 3. These are the NULL modes as used in the basic system, the
reference generator or RVG mode and the stabilizer or STAB mode. These will be
discussed in order.
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