Chopper amplifier

Potential drop across the current-measuring shunt was measured on a 0—100,000 fiv L N White double potentiometer potential drop across the test piece was measured with an L N Wenner potentiometer, using a calibrated potential divider to reduce the. signal by a factor of 101.003. The unbalance signal from each of the potentiometers was amplified by dc chopper amplifiers and recorded simultaneously on a two-channel Brush Mark II recorder so that the resistance of the test piece could be determined at any instant. 

The emf of the bulk liquid thermocouple was measured with a 10,000 juv L N White double potentiometer, using a dc chopper amplifier and strip chart recorder to record the potentiometer unbalance continuously on a chart annotated so that it could be correlated with the chart recording the test-piece resistance. 

Chopper amplifiers do have several disadvantages. Mechanical choppers have a short lifetime and are expensive. In general, this system is satisfactory for dc signals and slowly varying ac signals. Unless quite sophisticated circuitry is used, chopper amplifiers are not satisfactory for processing repetitive... 

Figure 7.18. Chopper amplifier circuits (a) mechanical chopper input, (b) electronic chopper, multivibrator driver, and amplifier (Ferris, 1965fi).

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. 

For each EA spectrum, the transmission T was measured with the mechanical chopper in place and the electric field off. The differential transmission AT was subsequently measured without the chopper, with the electric field on, and with the lock-in amplifier set to detect signals at twice the electric-field modulation frequency. The 2/ dependency of the EA signal is due to the quadratic nature of EA in materials with definite parity. AT was then normalized to AT/T, which was free of the spectral response function. To a good approximation [18], the EA signal is related to the imaginary part of the optical third-order susceptibility ... 

The basic experimental arrangements for photocurrent measurements under periodic square and sinusoidal light perturbation are schematically depicted in Fig. 19. In the previous section, we have already discussed experimental results based on chopped light and lock-in detection. This approach is particularly useful for measurement at a single frequency, generally above 5 Hz. At lower frequencies the performance of lock-in amplifier and mechanical choppers diminishes considerably. For rather slow dynamics, DC photocurrent transients employing optical shutters are more advisable. On the other hand, for kinetic studies of the various reaction steps under illumination, intensity modulated photocurrent spectroscopy (IMPS) has proved to be a very powerful approach [132,133,148-156]. For IMPS, the applied potential is kept constant and the light intensity is sinusoid-... 

Such measurement provides the magnitude of birefringence, but not its sign. In addition, identical transmission values will be observed for multiple birefringence orders, that is, whenever the optical path difference, dAn, becomes a multiple of X. The main interest of this method arises from its excellent time resolution, below 1 ms, that is readily achieved using a low-power (e.g., 5 mW) continuous-wave laser and a photodiode. If the sample is initially isotropic, it is possible to follow the birefringence order to obtain quantitative results. For improved accuracy, a second (reference) photodiode or a beam chopper and a lock-in amplifier can be used. 

Chopper is a rotating wheel whose function is to break the steady stream of light from the hollow-cathode-lamp into an intermittent light that gives rise to a Pulsating current into the photocell this current is duly amplified and recorded. 

Most common lock-in amplifiers can be operated at frequencies ranging from a few Hz up to 100 kHz. This fact is important in analyzing the temporal evolution of optical signals for example, fluorescence decay time measurements. Although this particular application of lock-in amplifiers is beyond the scope of this section, it is instructive to mention that this can be done by tuning the relative phase (the time delay) between the signal intensity and the reference signal provided by the chopper. 

The audio signal resulting from rf amplification and detection is amplified and detected in a phase sensitive manner by using the original modulation phase as a reference. Both mixer vacuum tubes 6S) and mechanical choppers are used. The resulting DC voltage is fed into a recorder. For sufficiently small audio modulation amplitudes the first derivative of the resonance absorption or dispersion results from the narrow-banding technique. 

Equipment. The spectra were recorded on a Beckman IR12 spectrometer in the absorbance mode, with low amplifier gain and slit widths smaller than 1.6 of the half-band width of the OH or OD bands. Under these conditions the apparent optical density of the OH bands could be reproduced within 0.5%. To avoid errors from sample emission at temperatures higher than 100°C, the spectra were scanned with the chopper between sample and detector disconnected. 

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