Preamplifier Input Considerations

In the circuit, Q represents the ac-coupling (dc-blocking) capacitor. Cj is the combination of shunt stray capacitance (from input cables, etc., to ground) and the dynamic input capacitance of the amplifier itself. jR, is the input resistance of the amplifier system and combines input bias resistance and dynamic input resistance of the active element in the amplifier. 

The actual signal which reaches the active circuitry of the preamplifier is modified from by the input circuitry and is given by (Laplace transform 

The input coupling circuit is effectively a band-pass filter or it can be considered as a differentiating circuit at high frequencies with the time constant Tj = and an integrating circuit at low frequencies with the time constant i2 = + CJ. 

When direct-coupled (dc) amplifiers are used, Q is replaced by a short circuit. In this case, theoretically, there is no signal distortion. The / ,Cj combination, however, does represent an electrical load on the biological signal source and at high frequencies, when the shunting effect of C, becomes important, there is attenuation of the high-frequency components. This is especially important in using micropipette electrodes, as described in Section 7.1.1. 

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The purpose of this chapter is to discuss certain important considerations in the design and use of preamplifiers, although a detailed presentation of circuit design techniques is beyond its scope. With microcircuits and chips becoming readily available at low cost, it will soon be unnecessary to design an amplifier system from scratch. There are, however, important considerations concerning input characteristics and dynamic response which are common to all amplifier systems. These are the topics which are presented here. 

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