Patent GB-778381-A (GB)
Title: Improvements in circuits for comparing the magnitudes and phases of steady-state voltages or currents in amplifying networks
IPC: G01R27/28
Filed: 1953-05-06
Abstract: 778,381. Measurement of amplifier gain, frequency response, phase, and potentiometric voltage. ROGERS, F. E. May 6, 1954 [May 6, 1953], No. 12678/53. Class 37. Apparatus for comparing the voltages or currents respectively existing between or traversing distinct pairs of input and output terminals of an electric amplifying network comprises a variable potentiometric circuit excited by the output voltage to develop a comparison voltage supplied to a null indicator also supplied with a voltage proportional to the input, derived either directly or from an input potentiometer ; the null indicator being adjustable to zero by variation of one or other of the potentiometers, and a unidirectional linear non-rectifying transmission network (which may comprise an amplifier) being connected between the input voltage source and the null indicator. In Fig. 2 an amplifier A has input terminals 1, 2 con. nected to a voltage source Es and output terminals 3, 4 energizing series resistances R1, R2 with voltage Vo ; R2 being shunted by capacitance C1 correcting for the amplifier phase shift and the voltage across R2, C1 being connected direct to null indicator D for comparison with the input voltage Es, which is connected to the null indicator over a non-rectifying unilateral transmission network N (which may be a unity gain zero phase shift amplifier) so that no feedback can exist between the output and the input of amplifier A) whose gain is determinable by adjustment of potentiometer R1, R2 for null. In Fig. 3 (not shown) the unilateral network may be a cathode follower amplifier, and in Fig. 4 the null indicator D is alternatively connectible by switch K1 between the amplifier cathode and either the output voltage across C1, R2 or the slider voltage of potentiometer S1 across the input voltage source Es, so that the gain of the amplifier may be determined. In Fig. 5 the input of the cathode follower amplifier is derived from the slider of a potentiometer S2 in series with an equal resistance R7 across the input voltage source Es ; the null indicator being switchable between the amplifier cathode and either the output voltage across C1, R2 or the voltage Es - across resistance R7, so that in the latter 2 position potentiometer S2 is adjustable so that the overall gain from the input terminals 1, 2 to the null indicator D is accurately ¢. The amplifier grid may alternately be connected directly to the input terminal and the cathode resistance may be replaced by a variable potentiometer whose slider is connected to the null indicator. In the potentiometers, resistance is replaceable by capacitance or inductance while the cathode follower circuits may introduce attenuation. In Fig. 7 the input of the amplifying network under test may be excited from the output 13, 14 of an oscillator whose frequency is variable in known manner by variation of the capacitances of a resistancecapacitance quadrature phase shift network (Fig. 6, not shown) ; the oscillator output also exciting the input 9, 10 of an identical phase shift network PSN whose capacitances are ganged for variation with 'those of the tuning network of the oscillator, so that at any frequency the input and output voltages are in quadrature. The input voltage is halved by a potentiometer of equal resistances R10, R11 to excite terminal 15 of an inductive or capacitative rotary phase shifter VPS, while the output voltage excites a phase splitter triode T whose antiphased outputs of magnitude ¢ the input voltage are alternately selectible by switch K2 to energize terminal 17 of the phase splitter, so that its output at terminals 18, 19 is variable from 0 degrees to Œ90 degrees with reference to the input V/0 degrees at terminal 15, and is applied to a low-distortion amplifier energizing an adjustable potentiometer S2, whose output is connectible to one side of the null indicator, and whose other side is connected over terminals 20, 21 to the output of the amplifying network under test as described above, so that the gain and phase angle of the latter are measurable without feedback by adjustment of the phase shifter VPS and potentiometer S2. The phase shifter is replaceable by a pair of potentiometers respectivly varying the voltages V/0 degrees and V/Œ90 degrees applied to the amplifier input.