simple analogue voltmeter

There are many occasions when an analogue voltmeter is superior to a digital voltmeter. In certain circum- stances, the swing of a needle across a scale reveals more about the response of, and trends within, a circuit than a number of digital spot measurements. The main trouble with multi- function meters, such as the trusty "Avo 8", is that they have a low input impedance that presents such a large load on the circuit being measured as to make the readings totally meaningless in some applications. It was decided, therefore, to see how the latest semiconductor devices could be used to provide the Avo 8 with a very high input impedance and a full scale range of 25 mV. Any text book will show that the classic way of achieving this is to employ an operational amplifier set to a suitable gain with a couple of resistors, and a further resistor to limit the current into the meter to prevent damage in the event of an excessively large input being applied. All circuits of this type incorp...