Differential gain

When the inhomogeneity in crystal quality is sufficiently small compared to carrier diffusion length, carriers diffuse into poor-quality areas due to the slope of the carrier density. (In fact, the actual mechanism may be more complicated, because the bandgap inhomogeneity exists as well.) hi this case, radiative efficiency is determined by the poor-quality areas, and consequently the sample exhibits relatively more homogeneous but poor radiative efficiency in total. This results in both higher transparent current density and lower differential gain. 

Before the AFM experiment is started, the scan parameters must be adjusted. First of all the tapping AFM menu must be chosen compared to CM, there are a number of changes the scan rate must be reduced (max. 1.0 Hz) and the gains are typically set to lower values (integral gain 0.5 proportional gain 1.0 differential gain 0.0). 

Gibbs, H.M., McCall, S.L., and Venkatesan, S.L. Differential gain and bistability using a sodium-filled Fabry-Perot interferometer, Phys. Rev. Lett. 36, 1135—1138 (1976)... 

Common mode rejection ratio (CMRR) CMRR of a differential amplifier is defined as the ratio between the amplitude of a common mode signal and the amplitude of a differential signal that would produce the same output amplitude or as the ratio of the differential gain over the common mode gain CMRR = GD/GCM. Expressed in decibels, the common mode rejection is 20 log 10 CMRR. The common mode rejection is a function of frequency and source impedance unbalance. 

Shimizu J, Yamada H, Murata S, Tomita A, Kitamura M, Suzuki A (1991) Optical-confinement-factor dependencies of the K-factor, differential gain, and nonlinear gain coefficient for 1.55 im InGaAs/InGaAsP MQW and strained-MQW lasca. TEF.F. Photonics Technol Lett 3 773-776... 

Higher differential gain coefficients can also be obtained by utilizing semiconductor quantum well strac-tures. Due to the confinement of carriers in a direction of the quantum well growth, the density of states become stepwise and increases the differential gain coefficient. This staircase density of states distribution increases the modulation bandwidth by a factor of two. It is expected that another factor of two increase in the modulation bandwidth will be obtainable with quantum wire stiuctures, which have a quasi-discrete density of states distribution. 

A medium with saturable absorption can also be used for the nonlinear medium. In this case, the resonance condition is always met as far as the wavelength is concerned, but the feedback from the second mirror, which is required for resonant transmission, is not obtained until sufficient light is present inside the cavity to bleach the nonlinear medium. Devices of this type can show differential gain and optical bistability but not optical limiting. 

The differential gain go must, in general, be calculated from a detailed model of the semiconductor band structure. However, it turns out that go is a very nearly linear function of the density of electrons Nc injected into the conduction band by the pump source. An empirical expression often used to represent the differential gain is... 

Equation (3) can be explained in simple terms as follows. The rate of change of the carrier density (i.e., dNc/dt) is equal to the rate at which new carriers are injected, minus the rate at which carriers are removed via nonradiative, radiative, and Auger recombination processes, minus the rate at which carriers are removed via stimulated recombination. Note that the rate of stimulated recombination is proportional to the differential gain given by Eq. (2). This is because the creation of a new photon by stimulated emission (i.e., the gain process) corresponds precisely to the loss of one carrier through stimulated recombination. 

Figure 3 shows the differential gain of a nominally 1550 nm SOA as a function of wavelength for three different values of injected current, calculated using the density-of-states method. The device has a band-gap energy corresponding to 1580 nm however, the gain peak... 

Whether in the electrical or optical domain, amplification is never obtained for free—invariably the gain is associated with some level of added noise. In the case of an SOA, the fundamental origin of noise is spontaneous emission. As has already been discussed in Section II.A, with reference to recombination processes, electrons and holes spontaneously recombine resulting in the emission of a photon over timescales on the order of a nanosecond. Since the process is random it results in optical noise emitted over the complete gain spectrum of the SOA. Indeed, it was Einstein who first showed, on the basis of fundamental thermodynamic considerations, that in any inverted medium, spontaneous emission must occur at a rate that is proportional to the differential gain provided by the population inversion. In other words, spontaneous emission noise is an unavoidable fact of life. 

By using differential gain for the real and reactive standards, a suitable range of measurement can be selected for each impedance component separately. This feature is incorporated in Figure 3.1.5. 

The instrumentation amplifier is a circuit configuration that potentially combines the best features desirable for biopotential measurements [8], namely, high differential gain, low... 

This means that the differential gain is as follows ... 

Fig. 4. Impedance converters for measuring e.m.f. of a cell, (A) using a voltage follower (the effective input resistance, equals the common mode resistance of an FET-type operational amplifier), (B) using voltage feedback (Reff R-Ao, where R is the input resistance, Aq is the OA gain without feedback), (C) by means of an instrumentation amplifier (typical differential gain A = 1 + lO /Re t)- Abbreviations OA, operational amplifier AZ OA, automatically zeroed OA lA, instrumentation amplifier S, shielding.

Differential gain distortion is a distortion of the amplitude information on a subcarrier in the presence of amphtude modulation of another subcarrier or main carrier. Differential phase distortion is a distortion of the phase information on a subcarrier in the presence of amplitude modulation of another subcarrier or main carrier. The former is measured in percent whereas the latter in degrees. 

CMRR (DB)-20 LOG, DIFFERENTIAL GAIN COMMON MODE GAIN/... 

FIGURE 10.284 The concept of common-mode rejection ratio (CMRR) for an active-balanced input circuit (a) differential gain measurement (b) calculating CMRR. 

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