Output resistance

The electrochemical cell is represented by the resistance Rwe of the interface WE electrolyte, the resistance Rohm of the electrolyte between the WE and tip of the RE, the resistance Rei of the electrolyte between the tip of the RE and the CE, the resistance Rce of the interface electrolyte CE, and the inner (source) resistance Rre of the reference electrode. R0ther represents the sum of resistances outside the cell, e.g., the output resistance of the amplifier, any resistances resulting from current monitoring and cables. The potentiostat amplifies the difference U+ - U- by a factor A in the range from 104 to 107 ... 

Takao H, Tanaka N, Sugiura M, Sawada K, Ishida M (2009) Non-linear fluidic integrated circuits realized by pneumatic-field effect transducers with controllable output resistance. In Proceedings IEEE 22nd international conference on micro electro mechanical systems, Sorrento, Italy, 25-29 January 2009, pp 503-506... 

Commercial electrochemical instruments that measure currents down to 1 pA are available. To reach the nA and pA current ranges, conventional electrochemical instruments frequently are used in concert with a current amplifier, which is a module consisting of a current follower and an inverter (Figure 15.7.1). This device is placed between the working electrode and the corresponding lead from the potentiostat (which usually is the input to another current follower, CF) (19). The amplification factor is Rf/Ro, where Rf is the feedback resistance of the first amplifier and Rq is the output resistance (to CF). 

The device characteristic in saturation does not exhibit a perfect current source characteristic as Vos is increased the current generally continues to increase with some rate. In silicon transistors this is to first order a manifestation of channel length modulation. While channel length modulation is theoretically possible in OFETs with a small L, parasitic parallel conduction paths are the primary contribute to the output resistance of OFETs in saturation. 

Negative feedback also causes the output resistance (/ ) to be reduced... 

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 ... 

For illustration, consider the circuit schematic of a simple functional model of an analog integrator depicted in Fig. 4.26. In reality, an integrated operational amplifier is built by means of a number of transistors. The macro-model in Fig. 4.26 reproduces the input-output behaviour of an operational amplifier. It is sufficiently accurate for low frequencies. Its parameters that can be tuned are the gain. A, the input resistance Ri, and the output resistance Ro- The measurement at internal nodes of a real integrated circuit requires special equipment such as a probe station. The output voltage Vo of a bonded and packaged operational amplifier chip can be measured at one of its pins and may be used for the detection of possible failures in the circuit [36]. 

The speed of the incident shock wave was measured with the help of piezoelectric pressure gauges situated in the lateral walls of the channel. In order to match the high output resistance of the piezoelectric gauges with the wave resistance of the long line, tension... 

The current regulator dynamic output resistance will be pq = rconstant current source in parallel with a 10-kS2 dynamic... 

For small-signal circuits the output resistance Vg of the MOSFET (Malik, 1995 Sedra and Smith, 1991) is important in limiting the gain of amplifiers. This resistance is related to the small-signal drain conductance... 

FIGURE 7.50 The Class A amplifier transistor can be modeled as a current controlled current source with internal output resistance R. When R = Rl, half of the collector power is dissipated in R and the other half in Ri, providing a maximum efficiency ijmax of 50%. This agrees with Eq. (7.31). Not shown are the matching circuits transforming r e and Rl to a standard impedance, such as 50 2. 

FIGURE 7.54 The Class AB or B amplifier may be modeled as having a measurable input resistance when driven and an internal output resistance that maintains the power efficiency at or below the theoretical maximum of 78.5%. For... 

FIGURE 7.56 The nonsaturated Class C amplifier may be modeled as a current controlled current source with a measurable input resistance when driven, and an internal output resistance determined by the power efficiency, ij Rmt shares power with J sl> but Rint oo when ij... 

FIGURE 7.60 ThesaturatedClassCamplifiermaybemodeledasacurrentcontrolledvoltagesourcewithameasurable input resistance when driven, and an internal output resistance determined by the power efficiency t] shares power... 

The output resistance is zero. Thus, the output voltage of an ideal op-amp is independent of the current drawn from the output. 

Input resistances are not infinite and output resistance is not zero. Input capacitance is not zero as well. 

An ideal op-amp has infinite-input and zero-output impedances. Figure 7.86 shows a model for an op-amp, which accounts for the finite- (but large) input resistances and the low- (but not zero) output resistance of a real op-amp. For a /uA741, the differential input resistance R is about 2 MS2, the common-mode input resistance Ricu is 100 Mf2, the input capacitance C, is 4-7 pF (including the capacitance of the op-amp package), and the output resistance Ro is about 75 f2. There are other parasitic capacitances such as capacitance between input and output that are not shown, but for many applications, Q is the most significant capacitance. 

Although the input resistances are in the megohm range and the output resistance is 75 f2, it appears that the input and output resistances depart considerably from the ideal values of infinity and zero, respectively. However, negative feedback around the op-amp in an op-amp voltage amplifier increases the input resistance and reduces the output resistance. For example, let us determine the output... 

FIGURE 7.86 A model for the op-amp which accounts for input impedances and output resistance. 

Note that Jipas is merely the output resistance of the passive network that is obtained when A is set to zero. We also note that Zo approaches zero as GB of the op-amp approaches infinity and that for noninfinite GB, Zq is a function of frequency since A is given by Eq. (7.108). 

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