Let us assume that we are required to amplify a small signal say of the order of millivolt and that the signal v(t) is one in which change in volt with respect to time i.e. dv/dt is very small. If the signals are periodic and its time period is many minutes or even hours in duration. Hence, such signal can not be amplified with RC coupled amplifier because such wave could not passed in coupling capacitor.
For this purpose value of coupling capacitor should be very large which is not practically possible. Instead it would be necessary to use direct coupling between stages with such direct coupled amplifier we would not be able to distinguish between a change in output voltage as the result of a change in input voltage or as the result of a drift in some active component. If the amplifier has high gain, even small changes in the operating point of the first stage, amplified by the remaining stages, might cause a large change in output.
In order to overcome this difficulty a simple method is used in which a signal is divided in to small parts or in otherworld, signal id called chopped. This chopped signal can be amplified using RC coupled amplifier. When this signal is being amplified then its shape is just like a modulated wave. A detector circuit is used to detect these modulated wave.
when this detected signal is passed in low pass circuit then separates amplified A.C signal and also removed the chopped portion.
A low frequency input signal is shown in figure 1(b). Suppose that switch S1 is being driven so that it is alternately open or closed. Then the signal Vi at the amplifier input will appear as shown in figure 1(c). When S1 is open Vi = V, and when S1 is closed Vi = 0. Note that the waveform Vi is a chopped version of the waveform V. Due to this reason a circuit consisting a resistance R and switch S1 is called a chopper.
In figure 1(c) we note that switch S1 play an important role to change the shape of input signal V in to Vi shape. Since, the frequency of operation of the switch is very large (typically 100 times) in comparison with the frequency of the signal V. Therefore no appreciable change take place in V during the interval when switch S1 is open. As the output obtained at the output of chopper circuit is equivalent to V but the waveform Vi is just like a square wave whose amplitude is proportional to V and its average value is shown as dashed lines and that is also proportional to the signal V. Actually, the waveform Vi is a square wave at the switching frequency amplitude modulated by the input signal and super imposed on a signal which is proportional to the input signal V itself.
The low frequency cutoff of the A.C amplifier is relatively high frequency square waves passed with small distortion while the signal frequency is well below the cutoff point. The output wave form of the amplifier is shown in figure 1(d). Since these waves are completely modulated, therefore the chopper is often called a modulator.
The signal V is again recovered with the help of capacitor C connected at the output and the switch S2. The switch S2 closes and opens in synchronism with S1. Hence during the interval T1, the negative extreme of VA is restored to zero, whereas during T2 the positive extreme is restored to zero. As a result, except for an increase in amplitude, the signal V0 across the switch S2 considers again the form of the signal Vi.
When this signal V0 is passed through a low pass filter which stops the square wave and allows the signal frequency to pass at the filter output. The combination of the capacitor C and the switch S2 play the roll of synchronous demodulator. The amplifier of figure 1(a) is called a chopper stabilized amplifier.
Different types of devices and circuits such as four diode gate and electro-mechanical relays are used as a chopper switches. But for the same purpose, transistors are to be used. The collector and emitter terminals of the transistors are to be used as the switch terminals. The switch is OFF by supplying a base current properly to drive the transistor in to saturation, and the switch on ON by applying a base current properly to drive the transistor beyond cutoff.
Figure 2 show the different circuits in which transistor is used as a chopper.
(a) Series type, normal transistor connection.
(b) Shunt type, normal connection.
(c) Series type, Inverted connection.
(d) Shunt type, inverted connection.
In figure (a) and (c), the input signals are in series while in figure (b) and (d), the transistor is in shunt across the normal manner, with the gating square wave VG applied between base and emitter. While in figure (c) and (d), the transistor is operated in the inverted manner with the gating signal applied between base and collector. The low frequency A.C waves are provided at VG. When VG source is OFF, then transistor remains in cutoff. But when VG source is ON, then transistor conduct. During the square wave when signal at zero, the transistor will not remain in conduction state. Just like this switching will be according to VG source frequency chopped output will obtain when switch is ON.
A bipolar transistor is preferred over the field effect transistor (FET) because of high speed when used as switching element. However, in slower switching application in chopper circuits, the FET find wide applications.
When a large reverse biasing voltage is applied to the FET gate, the device will be pinched off and only a small leakage current will flow between source and drain. The leakage current may be of the order of 1mA and the pinch off voltage required to reduce the current to this level may range from several volt to several tens of volts.
A FET chopper circuit is shown in Figure 3, in which the FET appears as a shunt element and should be compared with figure 2 (b) or (d). A gating signal is applied between gate and source to turn the transistor ON and OFF. The slowly varying input signal V appears at the chopper output as a chopped signal Vi as shown in figure 1.
FET chopper like a transistor chopper suffers from the leakage current which flows when the device is OFF. A method for compensating for the leakage current of FET is shown by the dashed portion. A square waveform is applied through the drive D. The levels of this wavefrom are selected to be such that when the FET is ON the voltage across D is small enough so that negligible diode current flows. On the other hand, when the FET is OFF the diode D is reverse biased and the reverse biased diode current supplies the leakage current of the FET. The voltage would be selected for proper compensation when the FET is OFF. When the FET is ON offset voltage will appear across the FET due to the diode leakage current. this offset voltage, however may be quite small. For assuming a leakage current of 10mA and an ON resistance of 500Ω, the offset voltage will be only 5µv.