Carrier and chopper amplifiers

Carrier and chopper amplifiers

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  Types of BioAmplifiers 2 Operational Amplifier Differential Amplifier Instrumentation Amplifier Isolation Amplifier Carrier Amplifier Chopper Amplifier
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  CARRIER AMPLIFIER ■ Adirect-current amplifier ■ The dc input signal is filtered by a lowpass filter, then used to modulate a carrier so it can be amplified conventionally as an alternating-current signal; ■ The amplified dc output is obtained by rectifying and filtering the rectified carrier signal. 3
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  ❖ Oscillator isused to energize the transducer with an alternating carrier voltage. Transducers require ac excitation and whose impedance is not purely resistive. ❖ Transducer shall change the amplitude of the carrier voltage in relation to changes in the physiological variable being measured. ■ Output of transducer would be an amplitude modulated signal. ■ Modulated signal is given to multi stage capacitance coupled amplifier ■ First stage produces amplification of AM signal ■ Second stage is constructed that it can respond only to signal frequency of the carrier. It can be further amplified in the following stage 4
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  ■ Then signalis demodulated in phase sensitive demodulator circuit. It helps to extract amplified signal voltage after filter circuit. The voltage produced by the demodulator can then be applied to the driver stage of the writing system. ❖ Carrier amplifiers can be used with a resistance strain gauge transducer such as a semiconductor strain gauge. ■ When used with pressure gauges, a calibration control is provided on the carrier amplifier. 5
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  Lock-in amplifier ■ Lock-inamplifier is a useful version of the carrier technique designed for the measurement of low-level signals buried in noise. ADVANTAGES: ■ It reduces wideband noise and increases the signal-to noise ratio. ■ This permits the signal to be recovered from its noisy background. 6
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  CHOPPER AMPLIFIER ■ Choppercircuit is used to refer to numerous types of electronic switching devices and circuits used in power control and signal applications ■ Chopper stabilizes a system against drift of electronic components 7
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  CHOPPER AMPLIFIER ■ Thechopper amplifier is a useful device in the field of medical field. ■ It gives another solution to the problem of achieving adequate low frequency response while avoiding the drift problem inherent in direct coupled amplifiers. 8
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  ■ This typeof amplifier makes use of a chopping device, which converts a slowly varying direct current to an alternating form with amplitude proportional to the input direct current and with phase dependent on the polarity of the original signal. ■ The alternating voltage is then amplified by a conventional ac amplifier whose output is rectified back to get an amplified direct current. ■ A chopper amplifier is an excellent device for signals of narrow bandwidth and reduces the drift problem. ■ Chopping the low frequency components of the input signal, amplifying this chopped signal in an ac amplifier (A1) and then demodulating the output of the ac amplifier 9
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  ■ Low frequencycomponents are derived from the input signal by passing it through the low-pass filter, consisting of R2, C2 and R2 ■ The filtered output is then further amplified in a second stage of dc amplification (A2). ■ Minor offsets and bias currents exist due to imperfect chopping, but these are extremely small. ■ Switch-driving oscillator built on the module 10
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  To avoid noise: ■It is desirable to design the feedback networks and external wiring to minimize the total circuit noise. ■ It is advisable that a feedback capacitor be used to limit the overall bandwidth and eliminate as much high frequency noise as possible. ■ Shielding of feedback components is desirable in chopper amplifiers ■ It is particularly necessary in electrically noisy environments. Use of shielded wire for summing junction leads is also recommended. Typical voltage drift in chopper-stabilized amplifiers is 0.1 mV/°C and current drift as 0.5 pA/°C. 11
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  Advantages ■ The greatstrength of the chopper-stabilized amplifier is its insensitivity to component changes due to ageing, temperature change, power supply variation or other environmental factors ■ Chopper amplifiers are available in both single ended as well as differential input configurations. 12
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  Applications ■ Used inmedical field in amplification of small dc signals. Such order of amplitudes are obtainable from transducers such as strain gauge pressure transducers, temperature sensors. ■ A chopper amplifier is also suitable for use with a thermocouple 13
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