Voltage Amplifiers

11-6660-204-25

produces an effective braking action, and prevents

greater than the slide-wire voltage. This causes an

overshooting the balance point.

ac error voltage to appear at the detector, which

acts to drive the slide-wire in a direction to bal-

ance out this voltage. As the slide-wire voltage

214. Voltage Amplifiers

rises toward the balanced condition, a charging

(figs. 2-12,2-13,2-14, and FO-3)

current is produced in capacitor C315 (developed

Five stages of voltage amplification are used to

by the movement of the slide-wire arm, dependent

increase the ac error voltage from the detector to

on its speed) which is in a direction opposite to

excite the power amplifiers, and drive the balanc-

that of the current produced by the original dc

ing motor. The first four stages are very stable,

e r r o r voltage. As the dc slide-wire voltage

because all operating voltages, including the fila-

changes to equal the dc applied voltage at balance,

ments, are obtained from regulated power sup-

the original current developed in capacitor C315

plies. The gain of the remainder of the servo sys-

and the error voltage developed in the detector is

tem varies inversely with the line voltage. Line-

reduced to zero. The opposing capacitor-charging

compensating feedback is incorporated in the am-

current is developed as long as the slide-wire is

plifier to compensate for line voltage variations to

moving; when the balance point is approached the

stabilize the overall gain of the servosystem. Fig-

opposing capacitor current will exceed the current

ures 212, 213 and 2-14 show all cathode resis-

that produces the error voltage. As this opposing

tors are unbypassed. This produces a degenerative

current is developed in the detector, an effective ac

circuit action and is done to further stabilize the

balancing voltage is developed in opposition to the

gain of the amplifier.

original ac error voltage; thus a reverse torque is

applied to the balancing motor for braking action.

a. When an unbalance occurs in the detector, ac

error voltage is developed in the secondary wind-

e. During any condition of unbalance, two op-

posing currents flow through the detector: one is

ing of transformer T302 and is applied to the grid

determined by the amount and direction of unbal-

of tube V308A, one-half of a twin-triode (fig. 2

ance, and the other by the speed and direction of

12). This signal is amplified and appears across

balancing response. Two opposing voltages are de-

plate-load resistor R338. Self-bias is provided by

veloped in the detector circuit: one tends to drive

cathode resistor R337, which is unbypassed to ob-

the slide-wire arm in one direction, and the other

tain negative feedback.

tends to drive it in the opposite direction. This

Figure 2-12. First and second voltage amplifiers, simplified schematic diagram.

2-15