Multistage D.C. Amplifiers

Author: J.B. Hoag

It is also possible to amplify d.c. voltages with multistage amplifiers, as in Fig. 24 E.

Fig. 24 E. Simple forms of multistage d.c. amplifiers

In the Loftin-White circuit of Fig. 24 E (d), a common battery is used for the plate and grid voltages. The filaments of the two tubes are at different voltages and require care in insulation from the ground. In all multistage d.c. amplifiers, the voltage available for the power supply must be comparatively great.

Fig. 24 F. A d.c. amplifier with a glow-tube in the inter-tube coupling unit

In Fig. 24 F, the glow-tube G takes the place of the grid condenser of an R-C coupled amplifier and acts as a constant-voltage device even for currents as small as 10 micro-amperes (10^-5 amp.) When R_g is 2 megohms (2 · 10⁶ ohms), the drop produced by 10^-5 amp. will be 20 volts. If R_c is chosen to give 30 volts, the net on the grid of tube 2 will be -10 volts. If the voltage at A changes, due to an input signal, the drop across G remains the same, but its current does change. Then the drop across R_g changes and is applied directly to grid 2. In other words, the circuit acts like an R-C coupled amplifier (see next chapter) with a leaky grid condenser which has the important characteristic that its voltage drop is always the same.