Feedback resistors

Blog Entry 3 A more simple solution to implement is to use the 3478 Low-side N-channel controller in a SEPIC configuration. This application utilizes two inductors (instead of a Flyback transformer) to attain the Buck-Boost function. The 3478 requires an external user-selectable Fet switch, so you can choose the one that suits your load current requirement. The datasheet provides an application rationale for SEPIC configuration on page 19, Figure 13. The output voltage can be set to 12V by changing the value of the feedback resistor. 

Incidentally, don t blindly add a bypass capacitor in parallel with the (upper) feedback resistor, as suggested. That feedforward capacitor introduces another zero in the loop and can cause the system to go unstable. You should realize that this family of devices has a full-blown internal Type 3 compensation, so it even has an internal zero to emulate an external ESR zero. That is why this family is supposed to be able to handle ceramic capacitors at the output. If you introduce yet another zero (via the feedforward capacitor as suggested), you could have one too many zeros. And ultimately, your design could be one, too (a zero). 

In order to allow the resistance measurement, the current injection is obtained in the circuit represented in fig. 3, by applying a voltage Vo- This current, due to the virtual ground condition determined by the circuit configuration (very high input impedance), will cross the feedback resistor Rf and determine an output voltage. In this example M and Yj (j=l,2,3) are the quantities ... 

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.

The larger the feedback resistor, the smaller the current noise. By using a 1 Mfi feedback resistor, the theoretical noise becomes 2 pA, a tangible value when very small tunneling current is measured. 

The noise of an actual resistance is always higher than the theoretical limit. While for metal resistors the noise level is close to the theoretical limit, the noise level in carbon resistors is much higher. The resistance of the tunneling junction, which is parallel to the feedback resistor, should be taken into account when its value is comparable to that of the feedback resistor. 

Fig. 11.2. Broad-band current amplifiers, (a) By replacing the feedback resistor in Fig. 11.1 with a resistor network, the cutoff frequency of the amplifier can be greatly increased, but the Johnson noise is increased, (b) Broad-band current amplifier with a compensation capacitor. By introducing a condensation capacitor C2, the effect of Q can be reduced. Under the condition CiRi = C2R2, the frequency range is substantially expended. The Johnson noise is not affected.

To make the entire eleetronic response linear with respect to tunneling gap s, a logarithmic amplifier is attached at the output of the current amplifier. A logarithmic amplifier can be made from a feedback amplifier, by replacing the feedback resistor with a diode, as shown in Fig. 11.4. The current-voltage characteristics of a good-quality, forward-biased silicon diode follow an exponential law over more than five orders of magnitude ... 

The Performance Analysis capabilities of Probe are used to view properties of waveforms that are not easily described. Examples are amplifier bandwidth, rise time, and overshoot. To calculate the bandwidth of a circuit, you must find the maximum gain, and then find the frequency where the gain is down by 3 dB. To calculate rise time, you must find the 10% and 90% points, and then find the time difference between the points. The Performance Analysis gives us the capability to plot these properties versus a parameter or device tolerances. Hie Performance Analysis is used in conjunction with the Parametric Sweep to see how the properties vary versus a parameter. The Performance Analysis is used in conjunction with the Monte Carlo analysis to see how the properties vary with device tolerances. In this section we will plot the bandwidth of an amplifier versus the value of the feedback resistor. See Sections 9.B.3 and 9.E to see how to use the Performance Analysis in conjunction with the Monte Carlo analysis. 

Suppose that for the example of Section 5.F, we would like to see a plot of how the upper 3 dB frequency is affected by the value of the feedback resistor, Rf. This plot can be accomplished using the Performance Analysis capabilities of Probe. Repeat the procedure of Section 5.F. When you obtain the plot on page 310, you may continue with this section. The plot on page 310 is repeated as follows ... 

EXERCISE 5-9 Plot the gain bandwidth of the amplifier in the previous example versus the feedback resistor RF. SOLUTIOIl Rerun the simulation with more detail in the Parametric Sweep ... 

Figure 4.1 Basic principles of a Faraday cup and amplifier circuit a) incoming charge of ions is converted into a voltage by an operational amplifier with a high ohmic feedback resistor (R) and b) dual ion detector for direct measurements of two ion currents ( and l2 using two amplifiers with resistors R, and R2.

Such amplifiers are commonly used with a feedback connection curve B of Figure 7.3 illustrates the relation between input and output for a ratio of feedback resistor Rfto input resistor Rj of Equation 7.10. Under this condition, a gain of -10 is obtained to about f = 10 kHz, beyond which the gain drops and the phase lag commences. The same results would be obtained with the control circuit of Figure 7.1 if Rs/Ru = 10 and if Cdl is shorted. Curve B can be calculated from the equation... 

Shaping the gain-frequency curve of the control OA requires knowing the value of feedback resistor Rf (Fig. 7.1) and an estimate of the cell roll-off frequency fs. First-order approximations for the stabilizing elements are... 

A very useful application of the circuit of Figure 8.3b is produced if we replace the feedback resistor Ru with a resistive transducer such as a coated semiconductor thermistor. The commercial availability of small, rapid-response, chemically inert thermistors that have conveniently measurable resistances at temperatures of 200-600 K makes them an excellent choice as the transducer in chemical applications that require the rapid and accurate measurement of temperature. Unfortunately, a continuous current in Ru may produce undesirable... 

A simple form of such a circuit is shown in Fig. 3. It uses a low noise operational amplifier and a feedback resistor (Rfb) with a typical impedance of R = 100 MQ. To get a maximum signal-to-noise ratio the bandwidth should be maximized while the noise should be as small as possible. 

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