Indian Institute of Technology Kanpur s1

Indian Institute of Technology Kanpur EE 381 Electrical Engineering Lab-II 2003 – 04; Semester II

Experiment #4 Study of Chopper Amplifier Goal Design and testing of a chopper amplifier used to amplify low-level signals. Introduction Several sensor outputs are DC signals in the microvolt to millivolt range. The DC amplifiers using opamps also have the input offset in the same range. At DC frequency, the drift of the amplifier also affects the measurement. One of the techniques used to achieve high precision dc gains with ac-coupled amplifiers is called chopper stabilization. In this technique, the input DC signal is modulated by a square wave, and an ac-coupled amplifier is used to amplify it. Further, it is synchronously demodulated and passed through a low pass filter to recover the amplified DC signal (Figure 1). Since the amplifier is ac-coupled, the input offset voltage does not appear at its output.

Chopping Signal v (t) v ch

0 t Low Amp Pass Output v v v v v 10mV 0.5V Filter 10mV 0.5V 0.23V -0.5V t t 0 t t t Figure 1: Principle of chopper amplifier The Fourier representation of the chopping signal of unit amplitude, vch ( t ) is given by the following equation:

v( t )=1 + ( 2 )sin( nw t ) ch2 np ch n=1,3,5⋯

If vi ( t ) is the input signal to be measured, the Fourier representation of the modulated signal can be written as:

v( t )=1 v ( t ) + v ( t ) ( 2 )sin( nw t ) m2 i inp ch n=1,3,5⋯ The modulation process translates the input signal to the odd harmonic frequencies of the chopping signal. The modulated signal is now amplified by an ac-coupled amplifier of gain A, and further demodulated back to the base band by the chopping signal. The output after demodulation can be represented by the following equation: ゥ v( t )= Av ( t ) (2 )sin( nw t ) ( 2 )sin( m w t ) d i邋 np ch m p ch n=1,3,5⋯ m = 1,3,5 ⋯ The output signal is recovered by passing the demodulated signal through a low pass filter which rejects all harmonic contents. The following figure (Figure 2) shows the entire process in frequency domain. The input signal is band limited to wi . The chopper frequency wch is chosen such that wi< w ch . The ac-coupled amplifier has a lower cut off frequency of wHP and the low pass filter cut off frequency is wLP .

V V V Vi Vm ma d o

w w w 3w 5w w w w w w i ch ch ch HP wLP

Figure 2: Principle of chopper amplifier in frequency domain

Multiplication of the input signal by a square wave is equivalent to switching the signal on and off. Therefore, a pair of switches, as shown in the following figure (Figure 3), can be used to realize the modulator and the demodulator. The two switches are driven by complementary control signals S and S'. The following circuit (Figure 4) consisting of two transistors can be used to generate the control signals.

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+5V S Rc1 Rc2 S1 Input Output S S' S2 Rb1 Rb2 S'

1kHz R1 -5V

Figure 3: Pair of switches used as modulator and demodulator Figure 4: Control signal generator

Pre-laboratory exercise Design the control signal generator circuit shown in Figure 3 meeting the following specifications.  The outputs S and S' are complement of each other.

 The range of high (VoH) and low (VoL) levels are: 4V < VoH < 5V and -5V < VoL < -4V.  Choose the resistance values such that the transistors operate in either saturation or in cut off modes.  Simulate the circuit using PSPICE and compare the simulated bias points with the design values.  Study the effect of driving the transistors too deep into saturation on the output of the circuit.  You will be using switching transistors 2N2369 in the laboratory to build this circuit. However, if you cannot find a model for this transistor, use the model of 2N2222 transistor for the simulation. Design an ac-coupled amplifier using the following schematic and meeting the following specifications:  Gain of the amplifier for the chopped signal is 100. R5  Explain the criteria for the choice of R1, C1, R2 and C2. R3 R4  Using PSPICE simulate the frequency response of the amplifier. - -  Plot the simulated output of the amplifier for a square + wave input of frequency 1kHz and having voltage levels + C2 Amp2 of 0V and 10mV. C1 Amp1 Design a first order low pass filter to extract only the base R2 band signal at the output of the demodulator. Explain the R1 criteria used to choose the values of resistor and capacitor. Set up the entire scheme of chopper amplifier on PSPICE. Use voltage controlled switch models for realizing the modulators if you cannot find a model for CD4066.  For an input signal set to 10mV DC, note down the signals at the output of (a) modulator, (b) Amp1, (c) Amp2, (d) demodulator and (e) low pass filter.  Plot the gain of the chopper amplifier as a function of the input voltage (Vi) for -50mV < Vi < 50mV.  Feed a low frequency low-level sinusoidal input to the chopper amplifier (say 20Hz at 60mV pp) and observe and sketch the output. Familiarize yourself with the specifications and pin outs of transistor 2N2369, CMOS analog switch CD4066, and operational amplifier ua741.

Laboratory Work Set up a potential divider network to obtain voltages in the range of -50mV to +50mV. Use +5V and -5V power supply and 10k pot to vary the voltage. Set up a direct-coupled non-inverting amplifier using ua741 to give a DC gain of 100. Plot the gain of the amplifier as a function of input voltage over the range of -50mV to +50mV. Take sufficient number of points to demonstrate the effect of DC offset as the input signal level approaches very small values. Do not use any offset nulling techniques to remove the DC offset. Set up the control signal generator (Figure 2) and verify the functionality of the circuit. Plot the waveforms at both S and S'. Set up the complete chopper amplifier circuit and for an input signal set to 10mV DC (which is set by the potential divider network), note down the signals at the output of (a) modulator, (b) Amp1, (c) Amp2, (d) demodulator and (e) low pass filter. Plot the gain of the chopper amplifier as a function of the input voltage (Vi) for -50mV < Vi < 50mV.

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Feed a low frequency low-level sinusoidal input to the chopper amplifier (say 10Hz at 60mV pp) and observe and sketch the output.

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