Bootstrapping techniques to improve the bandwidth of transimpedance amplifiers

Transimpedance amplifiers using voltage feedback operational amplifiers are widely used for current to voltage conversion in applications when a moderate/high bandwidth and a high sensitivity are required, such as photodiodes, photomultipliers, electron multipliers and capacitive sensors. The conventional circuit presents a virtual earth to the input and at low frequencies, the input capacitance is usually not a significant concern. However, at high frequencies and especially for high sensitivity circuits, the total input capacitance can severely limit the available bandwidth from the circuit. The input capacitance in effect constitutes part of the feedback network of the op-amp and hence reduces the available loop gain at high frequencies. In some cases a high input capacitance can cause the circuit to have a lightly damped or unstable dynamic response. Lag compensation by simply adding feedback capacitance is generally used to guarantee stability, however this approach does not permit the full gain-bandwidth characteristic of the op-amp to be fully exploited. Active techniques for reducing the input capacitance, for instance bootstrapping, have been previously reported and over the last ten years several papers have described successful examples of transimpedance circuits using bootstrapping. The basic bootstrapping principle is to use an additional buffer amplifier to actively charge and discharge to input capacitance as required. By doing so the effective source capacitance is reduced, enabling the overall bandwidth of the circuit to be increased. This paper presents a brief overview of the technique and introduces for the first time the four possible bootstrap configurations (series or shunt bootstrapping loops, with either floating or grounded sources) applied to the basic transimpedance circuit. Previous published techniques have employed the series technique, for example in the case of a photodiode bootstrapped by the source of the input FET. The shunt configuration has not been reported as in this paper. This work presents new analysis and practical results that demonstrate the shunt bootstrap configuration on a transimpedance circuit. A doubling of bandwidth over the standard transimpedance circuit has been realised using the same type of op-amp for both the amplifying and bootstrap elements of the circuit.