Patent GB-1490730-A (GB)
Title: Amplifier circuit
IPC: H03K5/02
Filed: 1974-11-28
Abstract: 1490730 Amplifying pulse signals THOMSON-CSF 28 Nov 1974 [30 Nov 1973] 51666/74 Headings H3P and H3W An amplifier for an intermittent unipolar signal has a series of directly connected amplifying stages A, a comparator D to compare the output signal amplitude to an external reference threshold V and a feedback path containing a low-pass filter C 1 , R 1  and a switch means, e.g. SW 1 , connected in series operated by the comparator D to open the feedback path when the level of the output signal exceeds the reference threshold. The result is said to produce a circuit in which the video signal is not differentiated and in which the D.C. component is correctly reconstituted. Normally the reference threshold is set higher than the noise level with the feedback path closed, i.e. with low overall gain from input E to output S and the capacitor G serves to remember the mean D.C. level of the no-signal condition. When a signal appears above the threshold the switch SW 1  opens and the system has high overall gain, the zero level being provided by C 1 . When the input has a D.C. component VE in the no-signal case this can be reduced to VS at the output by providing an amplifier H in the feedback path with a gain G=VE/VS. This need handle low frequencies and can have a very high gain, e.g. 100,000. To reduce the effects of the noise, which will also be amplified by amplifier H, another low-pass filter which may be an active integrator as shown, C 2 , R 2 , H, is provided, the amplifier H being of differential input type. An extra switch SW 2  controlled by the comparator D in synchronism with SW 1  is provided to prevent storage of the output signal in capacitor C 2  when SW 1  opens. When amplifier H and switch SW 2  are used, SW 1  could be omitted. In an alternative, Fig. 4, a differential amplifier F of unity gain is used with a switch SW 3  and low-pass filter C 3 , R 3  at its inverting input to store the output D.C. level of the amplifier in the absence of a signal and, by subtractive action, to ensure a zero voltage output level. Switch 3 is open in synchronism with switch SW1 at the onset of a signal. A simplified version (Fig. 5, not shown) takes the non- inverting signal from C 1 , thereby eliminating SW 3 , C 3  and R 3 . For correct operation on switching on the amplifier several methods may be used separately or in combination (Fig. 6, not shown). The switch in the feedback path may be temporarily shorted out by an auxiliary switch (X) for a predetermined time. A temporary/ permanent negative offset voltage (W) may be provided at an extra differencing input of the subtractor circuit (B). The comparator may be prevented from controlling the switch by inserting a circuit (Y) such as an AND circuit with one input connected to a delay circuit (not shown). The comparator itself may be inhibited by a delay circuit (Z) which can act on switches at the input or output of the comparator. If the pulses are very short the switches may not respond fast enough and the path may still be closed when the pulse has ended. This is said to be prevented by adding a delay circuit (Q, Fig. 7, not shown) which has a delay equal to the sum of the response times of the switches and the comparators. The switches may all be FETs. The invention may be applied in an otherwise conventional surveillance receiver (Fig. 8, not shown) where it would handle the video signals. A detector (Fig. 9, not shown) using a slightly forward-biased diode (40) can be used to derive the video signals which are amplified in a lownoise linear preamplifier (5) before application to the amplifier of the invention.