Amplifier input voltage

The voltage feedbaek loop must be isolated from the input voltage line and the eontrol IC. An optoisolator must be used. To minimize the drift effeet of the optoisolator an error amplifier is desired on the seeondary side. A TT431CP does this job nieely. The topology of the feedbaek eireuitry is shown in Figure 3-70. 

Electronic control A control system operating on low voltage, making use of solid-state components to amplify input signals from which the control functions are performed. 

Fig. 11.1. Two basic types of current ampliflers. (a) Feedback picoammeter. It consists of two components, an operational amplifier (op-amp) A, and a feedback resistor 1 fb- a typical value of the feedback resistor used in STM is 10 fl. The stray capacitance Cfb is an inevitable parasitic element in the circuit. In a careful design, Cfb 0.5 pF. The input capacitance Cm is also an inevitable parasitic element in the circuit. Those parasitic capacitors, the thermal noise of the feedback resistor, and the characteristics of the op-amp are the limiting factors to the performance of the picoammeter. (b) An electrometer used as a current amplifier (the shunt current amplifier). The voltage at the input resistance is amplified by the circuit, which consists of an op-amp and a pair of resistors R, and R2. The parasitic input capacitance Cm limits the frequency response, and the Johnson noise on Rm is the major source of noise. Also, the input resistance for this arrangement is large.

Another possible circuit is a voltage amplifier, with a shunt resistor to convert the input current to input voltage (see Fig, 11.1. This type of current amplifier has more disadvantages than the picoammeter The input... 

The center frequency is 1 kHz, This frequency was chosen to match the frequency of the sinusoidal input voltage. The harmonics that will be calculated are the first nine 1 kHz, 2 kHz, 3 kHz, 4 kHz, 5 kHz, 6 kHz, 7 kHz, 8 kHz, and 9 kHz. There may be others, but we want numerical values for only the first nine. The output variable for the Fourier analysis is the voltage at node Out, VfOUt). This is the output of the amplifier. We could look at the frequency components of any voltage or current, but for this example we are interested only in the output. Click the OK button twice to accept the settings and then run PSpice. 

In order to measure the frequency response of the class AB amplifier, the input voltage source was changed to a small-signal AC stimulus source (AC 1). The breadboard results are shown in Fig. 6.31. The IsSpice, PSpice, and Micro-Cap results are shown in Figs. 6.32, 6.33, and 6.34, respectively. 

Thus, the signal gain depends upon the ratio Rf/R. Figure 6.626 represents a buffer amplifier or voltage follower in which V0 = V,. This has the high input and low output impedances necessary to obviate the kind of inter-element loading problems illustrated in Section 6.11.6. For a more detailed treatment the reader is referred to Smith 881. 

The instrumentation amplifier is a high-performance differential amplifier consisting of a number of closed-loop op-amps. An ideal instrumentation amplifier gives an output voltage which is proportional only to the difference between two input voltages and Vtl, viz. ... 

Electrical measurements with both types of cells were made by using two devices as the gain-controlling elements of an inverting operational amplifier circuit which was driven with a constant input voltage (E. ). The ratio of the resistance of the reference device,... 

By choosing adequate amplifiers and using the feedback principle it is possible to construct circuits, making use of Ohm s and Kirchoff s laws to relate input voltage, Vj, with output voltage, V0. Some of the circuit components are illustrated in Fig. 7.6 with the respective relations indicated. The gain of these components must always be less than the gain of the OA at open circuit. 

If amplification of a small input voltage is desired, either of the circuits shown in Fig. 66 and 6c can be used. In the latter case, the noninverting input is grounded and the amplifier acts to hold the inverting (—) terminal at a virtual ground, i.e., = 0. (Actu-... 

The differential solution adds some additional circuit complexity in the form of auxiliary feedback loops to set the common-mode voltage at the amplifier input and output [17, 18]. 

This technique requires that the input voltage be sensed, and the slope of the comparator sawtooth ramp increased, if the input goes up. In the simplest implementation, a doubling of the input causes the slope of the ramp to double. Then, from Figure 7-11, we see that if the slope doubles, the duty cycle is immediately halved — as would be required anyway if the input to a buck converter is doubled. So the duty cycle correction afforded by this automatic ramp correction is exactly what is required (for a buck, since its duty cycle D = Vo/Vin)- But more importantly, this correction is virtually instantaneous — we didn t have to wait for the error amplifier to detect the error on the output (through the inherent delays of its RC-based compensation network scheme), and respond by altering the control voltage. So in effect, by input feedforward, we have bypassed all major delays, and so line correction is virtually instantaneous (i.e. perfect rejection of disturbance). 

In general, a zero input voltage e will not produce zero output voltage in a practical device. Instead there is a nonzero offset at the output. Most amplifiers have a provision for nulling the offset by an external adjustable resistor. 

FKsl.IRi 3-9 Response of an operalionat amplifier to a rapid step change in input voltage. The slope of Ihe changing portion of the output signal is Uie slew rate, and Ihe time required for the output to change from i0% lo 90% of the total change is the rise trme. 

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