Active Components in Preamplifiers

After amplification, the chopped signal can be converted back to dc by using a demodulator circuit which is simply a rectifier and low-pass filter. Mechanical demodulators are also used and consist of additional switch contacts actuated by the same armature which is used to chop the input signal. In this manner, synchronous operation is assured. 

Advantages of chopper amplifiers, when their use is applicable, are stable operation (no baseline drift) and low noise output. Most active electronic components (transistors or tubes) exhibit 1// noise, that is, electronic noise increases as frequency decreases and hence is largest at very low frequencies. If we convert a low-frequency or dc signal to the chopper frequency for amplification, then electronic noise produced in the active elements in the amplifier is greatly reduced. Operation of the chopper at 1 kHz would mean a thousand-fold reduction in noise of this origin at 1 Hz. 

The basic active devices which are used in preamplifiers in chronological sequence are tubes, transistors, field-effect transistors (FET s), and chips. Chips are self-contained microelectronic semiconductor circuits which are now available in numerous configurations. Thus one can buy certain basic amplifiers and other circuits as chips for incorporation into electronic systems. Much instrument design is now being done using these ready-to-use modular units. Many chips are comparable in size to transistors and have low power requirements. 

Bias techniques and design considerations are purposely omitted here. Those interested in such matters should consult standard texts on electronics, for example, that by Brophy (1972). 

The input voltages and are referenced to circuit ground. In the difference mode, this amplifier can be used to sense potential changes at one point in a circuit relative to another point in a circuit. A typical application is in EKG preamplifiers where two limb leads are connected to the input of a difference amplifier. They may also be used in null-indicator circuits to indicate equality 

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