A transistor d.c. amplifier for use in analogue computers

Abstract

The use of transistors in place of thermionic valves and electromechanical relays in d.c. amplifiers for analogue computers provides a potential means of reducing their size and power consumption and of increasing their reliability. The paper starts by analysing the transistor operational amplifier when used as a summing amplifier and as an integrator. This specifies the characteristics required to give a computing accuracy of better than 0.1% per stage in conventional real-time analogue computers.The amplifier designed to meet these requirements is in two parts. The first amplifier has a transfer impedance of 1000 volts/μA and gives an output swing of ±30 volts when feeding a 10-kilohm load. It is built up of five direct-coupled stages employing, where possible, feedback to minimize the effect of transistor parameter changes. Stabilizing networks are designed for overall resistive or capacitive feedback. The amplifier closed-loop bandwidth is 18 kc/s and the phase shift at 100c/s is less than 0.1° (for a 1-megohm feedback resistance).The second amplifier, which feeds from the virtual-earth point into the first, is a low-drift narrow-bandwidth d.c. chopper-type amplifier with a current gain of 500. In the complete system this reduces the drift of the first amplifier by a factor of 500 and gives a resultant input drift current, over a temperature range of about 5°C about room temperature, of 10−9 amp and over the range 25°–50°C of 10−8amp. With a 1-megohm feedback resistance the former leads to an outputvoltage drift of 1 mV.The overall bandwidth is extended to that of the first amplifier aloneby shunting the second narrow-band amplifier by an RC network. An analysis of the resulting double-loop amplifier enables the principal parameters to be chosen to maintain a stable system with the necessary gain over the range of operating frequencies.Some test results obtained on a printed-circuit version of the amplifier, which is to form the main computing element of a small generalpurpose analogue computer show the amplifier performance to beadequate for the majority of analogue computing applications.