The Differential Amplifier

Optical parametric oscillator (OPO, see 20) is the real equivalent to the radio frequency shifter however OPO can be replaced by a simple addition of a local oscillator (e.g. laser) through a beam splitter. Multiplication takes place at the level of detectors. For sake of S5mimetry, detectors can be placed at both output of the beam splitter, the intermediate frequency is then the output of the differential amplifier. 

The differential amplifier compares the attenuated input signal from the detector-amplifier network with the reference voltage, Vq, as illustrated in Figure 6.31. The resulting error signal, V- - Vq, is amp]ified and applied to the servomotor, causing it to move in one direction if the error voltage is positive. 

The output from the unity gain followers is input to two additional amplifiers. The first is a differential amplifier which subtracts the reference signal from the sample signal. The output of an array is a transmittance signal and if the reference is taken to indicate one hundred percent transmittance then the output of the differential amplifier will have a direct correspondence with transmittance. The second amplifier into which the sample and reference signals are input is a log-ratio-amplifier. The output of the amplifier can be considered as absorbance. 

Figure 6. Chip design and system setup for one-stage differential amplification. A DC voltage (Cl - F-) is applied to drive the particles from A to B. Trans-aperture voltage (Fdi and F02) modulation are sensed by the two gate branches to C and D, which are the positive and negative inputs of the differential amplifier, respectively. The resistances of the three sections in the main channel are denoted by Fi, R, R-i, respectively.

Fig. 11.20 (a) Analogue section of an automated PSA Instrument. The operational amplifiers are NS LF 336. The digital panel meter and the differentiating amplifier are optional, (b) Block diagram of the instrument. PI, parallel interface IRQ, interrupt request. Only 4 kb worth of the PROH capacity is used. Bus transceivers are not shown. (Reproduced from [95] with permission of Elsevier). 

Fig. 9.14. (Left) the differential amplifier. (Middle) and (right) the amplification of a varying signal and the input/output relationship for the differential amplifier. As is seen, the circuit shows a good linear response in a large regime of its total dynamic range. Many variations of the basic differential amplifier are possible. In a simpler circuit, the Ml transistor can be eliminated (resistor R1 then needs to be enlarged). For a single polarity input, the second gate can be connected to ground...

Periodic signals, either sinusoidal or square-wave can be used for many purposes. Here we present a few oscillator circuits based on electrochemical amplifiers. The first is the differential amplifier-based oscillator shown in... 

The positive lead of the single-fiber electrode (see Figure 25.6) is the end cap of a 25-/xm wire exposed through a side port on the cannula of a steel needle. Due to the small size of the positive lead, bioelectric sources, which are located more than about 300 /xm from the side port, will appear as common-mode signals and be suppressed by the differential amplifier. To further enhance the selectivity, the recorded signal is high-pass filtered at 500 Hz to remove low-frequency background activity from distant fibers. 

When trying to eliminate or minimize the effect of 60-Hz sources, it is sometimes useful to use a dummy source, such as a fixed 100-kS2 resistor attached to the electrodes. By employing a dummy source as one of the input signals, the output of the differential amplifier represents only contributions from interfering sources. If noise can be reduced to an acceptable level (at least by a factor of 10 less than EEG signals), it is likely that uncontaminated EEG records can be obtained. 

FIGURE 9.7 Equivalent circuit of an isolation amplifier. The differential amplifier on the left transmits the signal through the isolation barrier by a transformer, capacitor, or an optocoupler. 

Digital techniques also offer easy means of controlling the front end of the amplifier. Gain factors can be easily adapted, and changes of the electrode potential resulting from electrode polarization or from interferences that might drive the differential amplifier into saturation can be easily detected and compensated. 

Multisizer 4). The principle of the symmetric dual-channel design is to have the noise levels for the output signals (Vdi and Vd2 as indicated in Fig. 3) in both gate branches identical, and hence, the noises can be canceled by using a differential amplification mechanism. However, ideal noise subtraction is not possible due to the real-world limitation in fabricating identical dual channels. Furthermore, this dual-channel method will not be able to detect particles when two particles pass the two apertures at the same time because the two signals with the similar amplitude will be subtracted by each other and canceled at the second stage of the differential amplifier. 

The output resistive signals of the two detecting channels are connected to the differential -amplifier. When a particle passes the sensing gate, the theoretical output voltage difference between the two detecting channels amplified by the differential amplifier of gain A can be written as follows ... 

One additional point needs to be considered. The commercial DSC is constructed in a slightly different fashion. Instead of letting the differential amplifier correct only the temperature of the sample calorimeter by adding power, only half is added, and an equal amount of power is subtracted from the reference calorimeter. This is accomplished by properly phasing the power input of the two amplifiers. A check of the derivations shows that the result does not change with this modification. 

For a more detailed description of the working principle of the differential amplifiers, buffers, and power amplifiers shown in Fig. 3.6, the reader is referred to the excellent classic text on electronics by Horowitz and Hill [23]. 

Measurement of the ECG is performed using a differential amplifier. Two limb electrodes at a time are selected for the input to the differential amplifier stage. ECG amplifiers typically have gains of about 1000 and so increase the nominally 1-mV biopotential to about 1 V for driving a strip-chart recorder, cathode-ray-tube display, or computer data-acquisition card. Chart speeds in ECG recording have been standardized at 25 and 50 mm/s. 

The right-leg electrode is an impoitanl electrode even though it does not explicitly appear in the ECG Lead system. It provides the common reference point for the differential amplifier, and in older ECG machines, it is a grounding point for the body to reduce electric noise. 

A physically separate and remote reference electrode is desirable becau.se the reference electrode is not electrically isolated from the inputs to the differential amplifier. The reference electrode is in contact with the person and consequently is electrically coupled to both inputs of the amplifier by the electrical impedance of the body. This impedance is a function of distance, i.e., the further away the reference electrode is, the greater is the attenuation. The reference electrode forms a floating ground signal rather than a true ground. Furthermore, the noise signal that can be introduced by the reference electrode is not necessarily a conunon-mode signal that will be removed by the differential amplification process. 

A final comment about safety. The input lines from the electrodes in contact with the skin should be capacitively coupled to the inputs of the differential amplifier. The surface electrodes should not be directly attached to the differential amplifier inputs. The capacitive coupling blocks the flow of dc current from the amplifier to the user in the event of an electronic failure. If capacitive elements are not present to protect the user, then the user can suffer bums of the skin under the electrode site similar to those reported by Selvarajah and Datta (2001) for a myoelectric hand using a RSLSteeper single-site electrode. 

One final basic operational amplifier building block that is useful to present here is the differential amplifier it is illustrated in Figure 27.8. 

Finally, the second stage, which is the differential amplifier design considered earlier, provides additional gain determined by the ratio of the resistors R4 and R3, so we can describe the overall gain as... 

It was useful in some cases to use both systems (CVC and DA) in parallel. The resistor was short-circuited in one direction to allow high currents, recorded by the current-to-voltage amplifier, and very small currents in the other direction, recorded by the differential amplifier. Moreover, the differential amplifier could be modified into an auto-ranging system to record current transients after large potential steps [15]. 

---