Preamplifier impedance matching

Every electronic component has a characteristic impedance. When a signal is transmitted from a unit with a high-impedance output to a low-impedance input, there is going to be a loss in the signal unless an impedance-matching device is used to couple the two units. Manufacturers of preamplifiers and amplifiers quote the impedance of the input and output for their products. Coaxial cables have an impedance between 90 and 100 ft. 

Low noise figure preamplifiers with noise impedances matched and relatively near the receiver coils are required to avoid degrading SNR. Quadrature systems with preamplifiers on each channel have small SNR advantages over quadrature systems employing low-loss combiners and a single preamplifier. 

Vertical deflection—generates the vertical deflection voltages for the CRT. The input signal is impedance-matched and amplified by the Channel 1 and Channel 2 preamplifiers to a level suitable for driving subsequent stages. Channel selection switches operate under control of the microprocessor. 

Fig. 16. The conventional duplexer. Zq = 50 1. The characteristic impedance of the A/2 line from the branching point B to the preamplifier port is Z = 200 ft. The parallel capacitances of the crossed diode packages D, D2, D4, and are tuned by matched coils to 270 MHz that of Dj is tuned to 258 MHz. Note that package has a parallel resistor of 50 ft. D, consists of two pairs of BAW76 crossed diodes all other packages contain three such pairs.

The probe uses a single-coil for 20 mm spinning samples at room temperature. There are no tuning elements in the probe -- rather the tuning is by a variable inductor at the end of a A./4 cable as described in the discussion of quarter wave cables in V.C.8. The transformer at the end of the second quarter wave cable matches the impedance between the tank circuit and the broadband preamplifier. The output... 

Now at frequencies below a few hundred MHz, the typical capacitor used in NMR is virtually loss free and r is equal to the resistance of the wire in the coil and possibly of a cable between L and C. Since this is a small number, the series tank circuit requires additional circuitry to raise the impedance to match, for example, to a preamplifier with a 50 ohm input. [The need to present a certain impedance arises because the S/N of the preamplifier depends on this impedance Thus, this is an important matching condition.]... 

If fast signals with medium strength need to be recorded, then photodiodes are used in connection with so-called transimpedance amplifiers. In this case, the photodiode is driven in the photocurrent mode, i.e. it is driven in short-cut mode with Ud being nearly equal to zero. This mode requires that any preamplifier to be used is in transimpedance mode, i.e. its input impedance can be matched to that of the photodiode. This mode, also termed current-to-voltage converter, allows one to achieve low input impedance even for high gain. 

A is the reflected signal amplitude, is the incident signal amplitude, and Zq and Zj are the impedances on both sides of this point. Such condition is generated when cables of different impedances are joined or when the input resistor of the amplifier does not match the impedance of the signal cable. Let us discuss the consequences of Equation 143 in somewhat more detail. A preamplifier is connected to the Y-amplifier of an oscilloscope. The signal coming from the preamplifier travels in a coaxial cable with an impedance of Zq = 50 Q The input resistor of the oscillo-scopic amplifier is Z = 1 Inserting these values into Equation 143 yields for... 

Current mode preamplifiers can provide reliable counting up to 10 counts per second. They are specially suited for use with fission chambers.Voltage sensitive preamplifiers, with low gain, have high input impedance and low output impedance. They are useful for use with scintillation detectors, and also for matching of cable impedance. 

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