HU186189B - Arrangemenet for automatic frequency compensation of operational amplifiers - Google Patents

Abstract

The arrangement according to the invention is used to automatically compensate for operational amplifiers, in which a voltage divider for the amplifier output, a feedback resistor to the divider point and the inverter input of the amplifier, is output to a compensating capacity, and the other electrode of the compensating capacity is the amplifier. connected to a frequency compensation terminal. .. cibr c -1-

Description

Operational amplifiers are widely used in all fields of analogue and digital measurement technology. In digital measurement techniques, signal quantification and / or sample holder circuits, slip decoders for periodic or stochastic pulses, and integrators for most digital multimeters, are generally required prior to quantization.

As computers and especially microprocessors become more widespread, the digitization of measurement data and the automation of technological or measurement processes are becoming more common.

In these and other applications, it is often necessary to operationally adjust the gain of the operational amplifiers used. In the case of a measurement requiring a wide frequency band (eg pulse technique), it is problematic to properly compensate for the amplifier's frequency response. ¹

Along with changing the gain factor, also change the elements (elements) of the compensation network

- since they are usually connected to high-impedance points, only a significant reduction in bandwidth would be possible. (Compensation element kits for various amplifications, and extensive mechanical or electronic switching - would result in additional bandwidth reductions due to increased spreading capacity and increased high frequency interference sensitivity.)

The general method used is to have the compensation network for the lowest gain! they build the circuit and change only the gain factor. Figure I shows a Bode diagram of such an amplifier stage. (In the figure, A is the gain of the non-feedback amplifier, f, the breakpoint frequency corresponding to the dominant pole, f is the frequency of the second breakpoint and fj is the frequency set for the feedback gain of A _vl .)

- as shown in the figure - increasing the gain decreases the bandwidth proportionally and the phase reserve

- determined by the ratio of the breakpoint frequency of the feedback amplifier to the second breakpoint frequency - will be unnecessarily high. (The _amplitudes A _vl , A _v2 and A _v? Correspond to the _boundary frequencies f _vl , f _v2 and f _v3 respectively.)

In many applications, such a large decrease in the cut-off frequency cannot be tolerated.

Miklós Herpy: Analog Integrated Circuits, Technical Publishing House, Budapest, 1974. 143-158. She.

The arrangement according to the invention provides a simple solution to the problem described. When a suitable operational amplifier is selected, using the compensation method of the invention, the amplitude frequency and phase reserve of the amplifier automatically provide the maximum bandwidth when the amplification factor is changed.

The operation of the arrangement (for example, in the case of an inverting amplifier) is explained in Figure 2.

A commercially available (3554, 3551 Burr-Brown, AD 504 Analog Devices) or broadband operational amplifier having a frequency response of one output, coupled to an output K and one of the output capacitors for frequency compensation, can be provided. to change it.

Figure 1 shows a Bode diagram of an amplifier with a compensation network.

Figure 2 illustrates the proposed arrangement.

Figure 3 illustrates the Bode diagram of the proposed amplifier. .

Figure 2 shows an embodiment of the compensation according to the invention: the gain factor is varied according to the ratio of the output divider connected to the output. The scaling ratio is determined by the impedances Z and Z ₂ and the feedback resistance R ₂ . The gain can be conveniently controlled by varying the impedance Z ₂ .

The compensating capacity C _k for frequency compensation is not connected to terminal K, but is connected to the dividing point P by inserting a broadband follower F (such as a field-effect transistor) with a high input resistance. The figure shows the operational amplifier in two stages. Without compensation, assuming that the input resistance of amplifier stage _{2 is} high, the output resistance R of amplifier stage A and the internal capacitance C _b determine the breakpoint frequency of the dominant pole. (In Figures 1 and 3, the frequency of the breakpoint f is shown.)

Let us examine how, in the case of compensation according to Fig. 2, the breaking point frequency (i.e. gain) depends on the division ratio. In order to simplify the discussion, the effect of internal capacity C _b is neglected. (If the condition C _b <^ * ^J C _{k is} met, neglect is justified.)

It can be shown that in this case the breakpoint frequency (} '2-l) is ^f 2π RC _k (\ + aj'

If even the «Condition ₂ :

r ~ ¹ i '' 2 ~ 2π RC _k A ₂ ³ '

Because the.confirmation is:

Λ ^R >

_A. = _{s is given by} the relation, the frequency of the dominant pole is a linear function of the division ratio, ie the gain. Figure 3 shows a Bode diagram according to an arrangement according to the invention. The auto-adjusting breakpoint frequencies corresponding to the different feed-back amplifiers _Ávl , A _v2 , A _v , respectively, are f "f and f" and the cut-off frequency of the feedback amplifier is f _k , - f _k2 = f _k , - f _k , independent of the gain.

In practice, for large reinforcements, where the effect of internal capacitance C _b can no longer be neglected, on the other hand the condition <a, the break point is not met! frequency amplification dependence is described by a more complex relationship. (The appropriate breakpoint frequency is 21

186 189 (f, \ f, 'and f,' in Figure 3). In this case, the boundary frequency of the feedback system decreases with increasing gain. Of course, this decrease is always much smaller than what would occur with the normal setting of compensation (see Figure 1). As is readily apparent, the method of the present invention approximates the ideal efficiency the greater the amplitude compensation f, the break point frequency (f, may be greater than f ').

Claims (1)

A patent claim An arrangement for automatically adjusting frequency response of operational amplifiers, which in itself comprises (5) impulses (Z ,, Z.), an I / O bandwidth, a high input resistance broadband tracker (F), a compensation capacity (C _k ) and a feedback resistor (R ₂ ). characterized in that ^{J is} the voltage divider for the output of the amplifier stage (A), the feedback resistor (R ₂ ) for the splitter point (P) and the inverter input of the amplifier, the broadband follower (F) input for the splitter point (P), is connected to the compensation capacitance (C _k ) and to the compensating capacitance (Cj of the other electrode of Cj for amplifying frequency compensation).