CN204258730U - Voltage follower and voltage follower circuit - Google Patents

Abstract

The utility model discloses a kind of voltage follower and voltage follower circuit, belong to technical field of electricity, when solving existing voltage follower rear end connection capacitive load, easily self-oscillatory technical problem occurs.This voltage follower, comprises operational amplifier, isolation resistance, high frequency feedback passage and low frequency feedback passage; One end of described isolation resistance connects the output of described operational amplifier, and the other end connects the output of described voltage follower; Described high frequency feedback expanding channels is between the output and inverting input of described operational amplifier; Described low frequency feedback expanding channels is between the inverting input and the output of described voltage follower of described operational amplifier.The utility model can be used for the scenes such as the signal condition system of locomotive.

Description

Voltage follower and voltage follower circuit

Technical field

The utility model relates to technical field of electricity, specifically, relates to a kind of voltage follower and voltage follower circuit.

Background technology

It is high that voltage follower has input impedance, the advantage that output impedance is low, is therefore widely used in the scene such as signal condition system of locomotive.

As shown in Figure 1, voltage follower is primarily of operational amplifier U1 and feedback resistance R _fcomposition.Analog signal first by RC filtering, then is connected with back-end circuit by voltage follower, but the input of back-end circuit can be brought the noise of operational amplifier itself into, brings interference to back-end circuit.

Therefore, be directly connect capacitive load C in voltage follower rear end in a lot of scheme at present, as shown in Figure 2.But this voltage follower in the course of the work, be easy to self-oscillation occurs, affect the stability of voltage follower and whole system.

Utility model content

The purpose of this utility model is to provide a kind of voltage follower and voltage follower circuit, during to solve voltage follower rear end connection capacitive load, easily self-oscillatory technical problem occurs.

The utility model provides a kind of voltage follower, comprises operational amplifier, isolation resistance, high frequency feedback passage and low frequency feedback passage;

One end of described isolation resistance connects the output of described operational amplifier, and the other end connects the output of described voltage follower;

Described high frequency feedback expanding channels is between the output and inverting input of described operational amplifier;

Described low frequency feedback expanding channels is between the inverting input and the output of described voltage follower of described operational amplifier.

Preferably, described high frequency feedback passage comprises feedback capacity.

Preferably, described low frequency feedback passage comprises feedback resistance.

Preferably, the size of described isolation resistance is between 10 to 200 Ω.

Preferably, described operational amplifier is AD8628, AD8629, AD8630, OP4177 or OPA4227.

The utility model also provides a kind of voltage follower circuit, comprises output capacitor and above-mentioned voltage follower;

One end of described output capacitor connects the output of described voltage follower, the other end ground connection of described output capacitor.

The utility model brings following beneficial effect: in the voltage follower circuit that the utility model provides, output capacitor is connected at the output of voltage follower, namely directly connect capacitive load at the output of voltage follower, to filter out the noise of operational amplifier itself.Further, by arranging high frequency feedback passage and low frequency feedback passage in voltage follower, improve the stability of voltage follower, thus avoid voltage follower generation self-oscillation, enable whole system keep steady operation.

Other features and advantages of the utility model will be set forth in the following description, and, partly become apparent from specification, or understand by implementing the utility model.The purpose of this utility model and other advantages realize by structure specifically noted in specification, claims and accompanying drawing and obtain.

Accompanying drawing explanation

In order to be illustrated more clearly in the technical scheme in the utility model embodiment, accompanying drawing required in describing is done simple introduction below to embodiment:

Fig. 1 is the schematic diagram of existing voltage follower circuit;

Fig. 2 is the schematic diagram of another kind of existing voltage follower circuit;

Fig. 3 is the schematic diagram of the voltage follower circuit that the utility model embodiment provides;

Fig. 4 is the equivalent schematic of the voltage follower circuit that the utility model embodiment provides.

Fig. 5 is the Bode diagram of the voltage follower circuit that the utility model embodiment provides.

Embodiment

Describe execution mode of the present utility model in detail below with reference to drawings and Examples, to the utility model, how application technology means solve technical problem whereby, and the implementation procedure reaching technique effect can fully understand and implement according to this.It should be noted that, only otherwise form conflict, each embodiment in the utility model and each feature in each embodiment can be combined with each other, and the technical scheme formed is all within protection range of the present utility model.

The utility model embodiment provides a kind of voltage follower and voltage follower circuit, can be applicable to the scenes such as the signal condition system of locomotive.

As shown in Figure 3, the voltage follower circuit that the utility model embodiment provides comprises voltage follower and output capacitor C _l.C _lone end connect the output V of voltage follower _oUT, C _lother end ground connection.

Voltage follower comprises operational amplifier U1, isolation resistance R _iSO, high frequency feedback passage FB2 and low frequency feedback passage FB1.Wherein, operational amplifier U1 specifically can adopt the devices such as AD8628, AD8629, AD8630, OP4177 or OPA4227.

The in-phase input end of operational amplifier U1 is the input V of voltage follower _iN.R _iSOthe output of one end concatenation operation amplifier U1, R _iSOthe other end connect the output V of voltage follower _oUT.

Between the output that high frequency feedback passage FB2 is connected to operational amplifier U1 and inverting input.In the present embodiment, high frequency feedback passage FB2 comprises feedback capacity C _f.

Low frequency feedback passage FB1 is connected to the inverting input of operational amplifier U1 and the output V of voltage follower _oUTbetween.In the present embodiment, low frequency feedback passage FB1 comprises feedback resistance R _f.

In the voltage follower circuit that the utility model embodiment provides, at the output V of voltage follower _oUTdirectly connect C _l, this voltage follower circuit can be considered as a single pole low pass filter, and it is input as V _iN, export as V _oUT, its cut-off frequency is:

$f_{-3\mathrm{dB}}=\frac{1}{2\mathrm{\π}{R}_{\mathrm{ISO}}{C}_L}$

Therefore, this voltage follower circuit can the input signal of filtering simultaneously V _iNwith the noise of operational amplifier U1 itself.

When analyzing the stability of this voltage follower circuit, operational amplifier U1 can be decomposed into an an ideal operational amplifier U and open-loop output resistance R _ocombination, as shown in Figure 4.Wherein, the open loop amplitude frequency curve of ideal operational amplifier U is A in Bode diagram _oL, as shown in Figure 5.

In the low-frequency range of this voltage follower circuit, C _fopen circuit can be considered as, by expression formula 1/ β _l=V _oA/ V _fB, try to achieve transfer function 1/ β _l, β _lfor low frequency feedback coefficient.This transfer function 1/ β _lcomprise one zero point f _lz1.

$f_{\mathrm{Lz}1}=\frac{1}{2\mathrm{\π}(R_O+R_{\mathrm{ISO}})C_L}$

Wherein, R _iSOsize can be chosen for and R _oclose or equal, usually can choose between 10 to 200 Ω.C _lthe large I purposes concrete according to voltage follower circuit choose.As shown in Figure 5, when frequency is less than f<f _lz1time, 1/ β _l=1, its corresponding logarithmic coordinates 0dB.

At the high band of this voltage follower circuit, corresponding simplification can be carried out to voltage follower circuit.Usual C _flarge I be designed to much smaller than C _l, so in the high frequency stage, C _lcan at C _fbe shorted before.Therefore, C _leliminate low frequency feedback passage FB1 after short circuit, thus be convenient to independent analysis of high frequency feedback path FB2.By expression formula 1/ β _h=V _oA/ V _fB, can in the hope of transfer function 1/ β _h, β _hfor high frequency feedback coefficient.This transfer function 1/ β _hcomprise a limit f _hp2with one zero point f _hz2, wherein f _hp2=0, f _hz2as shown in the formula.

$f_{\mathrm{Hz}2}=\frac{1}{2\mathrm{\&pi;}{R}_F{C}_F}$

As frequency f >f _hz2time, 1/ β _hbe calculated as follows formula.

$\frac{1}{{\mathrm{\&beta;}}_H}=\frac{R_O+R_{\mathrm{ISO}}}{R_{\mathrm{ISO}}}$

According to the analysis of the above zeros and poles to this voltage follower circuit, Bode diagram as shown in Figure 5 can be drawn out, comprising amplitude frequency curve figure and phase frequency curve figure.In amplitude frequency curve figure, A _oLfor the open loop amplitude frequency curve of operational amplifier, f _p1and f _p2for the limit of operational amplifier itself, f _cfor the bandwidth frequency of voltage follower circuit.Amplitude frequency curve 1/ β of voltage follower circuit feedback path is by 1/ β _hwith 1/ β _larticle two, amplitude frequency curve forms, because the path of feedback factor β comparatively large (namely 1/ β is less) accounts for leading role, so 1/ β obtained as shown in broken lines.

When the open loop amplitude-frequency curve A of operational amplifier _oLtime crossing with the slope of-20dB/Decade with 1/ final β, on the relevant position of this intersection point in phase frequency curve figure, the phase margin γ >45 ° of voltage follower circuit, then show that this voltage follower circuit (connecting the voltage follower of capacitive load) is stable, this voltage follower circuit circuit can not produce self-oscillation in the course of the work.

Therefore, in the voltage follower circuit that the utility model embodiment provides, directly connect capacitive load at the output of voltage follower, thus can the noise of filtering input signal and operational amplifier itself.And, in the voltage follower that the utility model embodiment provides, by arranging high frequency feedback passage and low frequency feedback passage, improve the stability of voltage follower, thus there is self-oscillation when avoiding voltage follower connection capacitive load, enable whole system keep steady operation.

In addition, in the design process of voltage follower circuit, avoid the appearance of the G point in Fig. 5, this G point is 1/ β _hwith 1/ β _lthe intersection point of ＇.Wherein, 1/ β _l＇ is due to C _lbigger than normal, and cause 1/ β _lthe result that offsets left at zero point, simultaneously owing to compensating the too little appearance causing G point.When the frequency of voltage follower circuit is in this G point time, the closed loop gain of operational amplifier can sharply increase, and to whole circuit, especially may cause negative effect to operational amplifier.Therefore, in order to the absolute stability of the voltage follower circuit of retainer belt capacitive load, avoid the problem of G point, should f be made _hz2≤ f _lz1, that is:

$C_F\&GreaterEqual;\frac{R_O+R_{\mathrm{ISO}}}{R_F}{C}_L$

Or, at the open-loop output impedance R of operational amplifier _owhen unknown, can, according to the design experiences of this area, adopt following formula to estimate, also can keeping system stablize.

C _L×R _ISO＜0.5×R _F×C _F

In design voltage follow circuit process, determining C _land R _iSOafterwards, just C can be selected easily according to above relational expression _fand R _f, during general actual value, choose conventional C _fand R _f.

Although execution mode disclosed in the utility model is as above, the execution mode that described content just adopts for the ease of understanding the utility model, and be not used to limit the utility model.Technical staff in any the utility model art; under the prerequisite not departing from spirit and scope disclosed in the utility model; any amendment and change can be done what implement in form and in details; but scope of patent protection of the present utility model, the scope that still must define with appending claims is as the criterion.

Claims (6)

1. a voltage follower, is characterized in that, comprises operational amplifier, isolation resistance, high frequency feedback passage and low frequency feedback passage;

One end of described isolation resistance connects the output of described operational amplifier, and the other end connects the output of described voltage follower;

Described high frequency feedback expanding channels is between the output and inverting input of described operational amplifier;

Described low frequency feedback expanding channels is between the inverting input and the output of described voltage follower of described operational amplifier.

2. voltage follower as claimed in claim 1, it is characterized in that, described high frequency feedback passage comprises feedback capacity.

3. voltage follower as claimed in claim 1, it is characterized in that, described low frequency feedback passage comprises feedback resistance.

4. voltage follower as claimed in claim 1, it is characterized in that, the size of described isolation resistance is between 10 to 200 Ω.

5. voltage follower as claimed in claim 1, it is characterized in that, described operational amplifier is AD8628, AD8629, AD8630, OP4177 or OPA4227.

6. a voltage follower circuit, comprises output capacitor and the voltage follower as described in any one of claim 1 to 5;

One end of described output capacitor connects the output of described voltage follower, the other end ground connection of described output capacitor.