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Transimpedance Amplifier with Improved Speed, Sensitivity, and Dynamic Range

IP.com Disclosure Number: IPCOM000048297D
Original Publication Date: 1982-Jan-01
Included in the Prior Art Database: 2005-Feb-08
Document File: 2 page(s) / 35K

Publishing Venue

IBM

Related People

Moore, VS: AUTHOR [+2]

Abstract

The loss of sensitivity, in a tradeoff with speed end dynamic range requirements, has been reduced by improvements added to the basic transimpedance amplifier circuit.

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Transimpedance Amplifier with Improved Speed, Sensitivity, and Dynamic Range

The loss of sensitivity, in a tradeoff with speed end dynamic range requirements, has been reduced by improvements added to the basic transimpedance amplifier circuit.

The basic transimpedance amplifier is shown in Fig. 1. The sensitivity is limited by thermal noise from resistor RFA and shot noise from the base current of QIA. The gain is approximately equal to RFA. The dynamic range is theoretically limited by noise on the lower end and by the amount of signal current which causes the output voltage (VOUT) to approach ground at the upper end. In a practical application, the maximum current signal is limited to a lesser value.

This assures that overshoot does not drive QIA into saturation and Q2A into cutoff, which would greatly "slow up" the circuit. Thus, resistor RFA influences the lower and upper limits of the input signal currents.

The improvements to the basic circuit are shown in Fig. 2. The addition of Q3B, between QlB and RLB, forms a 'cascode' circuit.

This reduces Q1B's input capacitance, and improves the circuit speed. The addition of Q5B, between RLB and Q2B, provides current gain, allowing the value of RLB to be increased, without loss of bandwidth.

This, in turn reduces the collector and base currents in Q1B, reducing its attendant shot noise and improving circuit sensitivity.

The addition of Q4B and RCB prevents saturation of the input stage (Q3B and Q1B) and cutoff of t...