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Transimpedance Input Stage for Current Amplifiers

IP.com Disclosure Number: IPCOM000043758D
Original Publication Date: 1984-Sep-01
Included in the Prior Art Database: 2005-Feb-05
Document File: 2 page(s) / 28K

Publishing Venue

IBM

Related People

Rogers, RL: AUTHOR

Abstract

Combining the transimpedance amplifier, which has the advantages of linearity, low noise and low effective input impedance, with the current amplifier, which is capable of very large gain-bandwidth products, onto a single chip reduces high input noise levels. Fig. 1 shows a schematic diagram of a conventional transimpedance amplifier. For this type of amplifier, it is desirable to operate with a relatively large transimpedance to obtain a low noise level and a large output voltage swing. However, if the output 11 of this type of amplifier, is connected directly to the low impedance input of a current amplifier the output stage is not able to sustain the relatively large voltage swing at the output which exists when a large transimpedance is used. Fig. 2 shows the improved transimpedance amplifier.

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Transimpedance Input Stage for Current Amplifiers

Combining the transimpedance amplifier, which has the advantages of linearity, low noise and low effective input impedance, with the current amplifier, which is capable of very large gain-bandwidth products, onto a single chip reduces high input noise levels. Fig. 1 shows a schematic diagram of a conventional transimpedance amplifier. For this type of amplifier, it is desirable to operate with a relatively large transimpedance to obtain a low noise level and a large output voltage swing. However, if the output 11 of this type of amplifier, is connected directly to the low impedance input of a current amplifier the output stage is not able to sustain the relatively large voltage swing at the output which exists when a large transimpedance is used. Fig. 2 shows the improved transimpedance amplifier. The circuit is similar to the conventional transimpedance amplifier with the difference being that the output of the transimpedance amplifier is not used as an output at all. Instead, an additional transistor T3 is added with its emitter and base parallel to transistor T1. Such a connection forms a "current mirror" in which the collector currents of the two transistors T1 and T3 are always proportional. The output 12 of is a current source and can be very simply coupled to the input of a current amplifier. With this arrangement, the current gain of this input stage is:

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where A is the ratio of the emitter...