DE1537590A1 - Differential amplifier circuit - Google Patents

Description

5407

G. & E, Bradley, London / England

Differential amplifier circuit

The invention relates to a differential amplifier circuit, which contains a differential amplifier with two input connections and two output connections.

The two input connections of the differential amplifier are symmetrical about a common point belonging to the amplifier. This common point is common Earth or mass. The signal to be amplified is applied between the input terminals. If two different Signals are to be compared with each other, they are applied separately between the common point and one of the input terminals.

When a signal is between the common point and each is applied to one of the input terminals at the same time, this signal is called a common mode signal. A Common mode signal can be in a source one between the The signal applied to the input terminals occurs when the internal impedance of the source is not relative to the common Point is symmetrical. Furthermore, if a differential amplifier is not completely symmetrical in Beaug on the common Point (balanced to earth) is formed, an attenuated common-mode input signal appears at the output of the Amplifier even if the input signal source is balanced is.

909884/0713 _ba d original

For example, if the differential amplifier is an arithmetic amplifier with an input resistor each in each input circuit and two feedback resistors, and the amplifier should have a high input impedance, the input resistances can be, for example have a face value of 1 Mfi. The value of this Resistances can differ by about 10 kSL, bo that Currents caused by the common-mode signal and flowing through these resistors to the undesired input signal can generate that is of the same order of magnitude as the useful signal that is to be amplified. That I cannot develop complete symmetry in practice, a common-mode noise signal at the output is unavoidable.

An object of the invention is therefore seen in the Common mode signal suppression in differential amplifiers to improve.

ACCORDING TO ^"1 ^ r invention this object is achieved in that a bypass amplifier (by-pass amplifier) is provided which is so constructed and arranged that it supplies the output terminals Ausgleiohssignale which are substantially identical to the common mode signals when this in a predetermined part of the differential amplifier occur.

So that the compensation signal is largely identical to a GM clock signal, the compensation signal must when the common mode signal has a DC component contains, also contain a direct current component which is largely the same in magnitude and sign as

909884/07 13

BATH

Is a direct current component of the common mode signal, and if the common mode signal is an alternating current component contains, the equalization signal must also have an AC component contain the amplitude of which is largely equal to the amplitude of the alternating current component of the Common mode signals and which are largely in phase with is the AC component of the common mode signal.

Because the by a common mode signal and the equalization signal evoked potentials of the input and output connections are largely the same, flows no common mode current in a predetermined part of the amplifier. For example, the mentioned computational amplifiers ensure that there are no asymmetricals via the input resistances Currents flow more.

In practice, the common mode signal now appears on Amplifier output, since the equalization signal is fed to the output terminals, however, the ratio is of the amplified useful signal to the common mode signal is now improved by a factor equal to the gain of the amplifier is. In addition, the output impedance of the amplifier can be made very low, e.g. 1 ohm, and the input resistances of a downstream Differential arithmetic amplifiers can be selected to have a high resistance, so that each Asymmetry is practically negligible and the common mode signal at the output of this downstream Amplifier can be easily suppressed.

909884/0713

As mentioned earlier, it can be "with the amplifier be an arithmetic amplifier, and moreover it can be several Current transfer stages (also known as emitter-coupled or collector-coupled, push-pull stages) with field-effect transistors or conventional transistors, also called "transistors".

The terms "cathode", "anode" and "control connection" are generally used here instead of current output connection (Source) and emitter connection, suction electrode connection (sink) and collector connection and gate and base connection of FieldL-Effect transistors and transistors are used. The transistors and field effect transistors will continue also referred to as active components.

The diversion amplifier can be an isolation amplifier act with gain factor 1, between the cathodes of a first power take-off stage and the Anodes of a second current transfer stage is connected. The control connections of a third current transfer stage, whose active components are connected as cathode followers are, and which forms the amplifier output stage, can with the anodes of the active components of the second current transfer circuit be connected.

So the common mode signal becomes practical to the output via the cathode follower in the first current transfer circuit, via the amplifier with gain factor 1 and supplied via the cathode follower in the third current transfer stage.

09884/0713

BATH

A fourth flow transfer stage with two active components can be placed in the anode circuits of the active components the first power takeover stage to be switched to a Difference between the amplitude of the common mode signal at the control connections and cathodes of the active components of the first current transfer stage. The control connections of this fourth current transfer stage are connected to one another and via a further isolating amplifier with gain 1 connected to the cathodes of the active components in the first current transfer stage, to the common mode further improve signal suppression.

The two isolating amplifiers Ic can be connected in series with the first-mentioned isolation amplifier between the output of the other isolation amplifier and the anodes of the two active components of the second current transfer stage.

Embodiments of the invention will now be based on the Drawings described in more detail.

In the drawings is:

Pig. 1 is a block diagram of a circuit with a Differential amplifier,

2 shows a circuit diagram of a differential amplifier according to the invention and

Fig. 3 is a circuit diagram of a further embodiment of a differential amplifier according to the invention.

909884/0713

The one in Pig. 1 includes one or more amplifier stages 10, input resistors R ₁ and R ' _1f, feedback _{resistors R 2} and R ¹ P and single-circuit connections 11 and 12. The signal to be amplified is from a source 13, which is between the input connections, and A clock input signal from a source 14 is supplied to input terminal 11 and via source 13 to input terminal 11. There is a consumer 15 at the output of the amplifier. There are four circuits for the output current of the current source 14:

1) via the resistors R ₁ and Rp and the consumer 15 to ground

2) across the resistors R '.. and R'p and the consumer 15 by mass

3) via the resistor R ₁ and the amplifier stages 10 to the output terminal 16 and

4) via the resistor R ¹ .. and the amplifier stage 10 to the output terminal 17.

The current flowing from the current source 14 via the resistors R ₁ and R'- .. causes a voltage at the input of the amplifier stages 10 if these resistances are not exactly the same. This additional input voltage can be large in relation to the signal from the source 13 if, for example, the resistors R ₁ and R- '. to 1 megohm resistors and the resistors R ₂ and R _'2 is 100 megohm resistors, as an asymmetry for example, 10 kilo-ohms between the resistors R ₁ and R is probably ¹ ₁ and a common mode voltage of only 10 mmV a Input voltage of microvolts.

909884/0713

BAD ÖHiQi _NAL

One from source H via resistors R- and R '^ Current flowing can be suppressed by connecting to the output terminals 16 and 17 of the amplifier an equalization signal is applied which is identical to the common mode signal. For example, if the common mode signal is a direct current signal, there is no voltage that would drive a current through the resistors R., because the compensation signal at connections 16 and 17 according to amount and sign is equal to the common mode signal at the terminals 11 and.

The common-mode signal is supplied to the input connections of the load 15, but the ratio is of the amplified output signal of the amplifier stages10 improved to common mode sign ^ l. In addition, since this ratio is improved by a factor equal to Gain of the amplifier is, and since the output impedance of the amplifier stages 10 are kept low can, for example, to 1 ohm, is the common mode rejection at consumer 15, assuming that this consumer is doing another amplifier acts, relatively easy.

A possibility of supplying equalization signals to the output connections in order to improve the suppression of common-mode signals which arise in that part of the amplifier which is formed by the input resistors R ^ and R ¹ .., as a result of an inequality of R-. and R ¹ .. or an imbalance in one of the input terminals

909884/0713

BATH ORIGINAL

11 and 12 connected source, is in Pig. 2 shown. The amplifier stages 10 together with the resistors * R-, R ¹ -, R ₂ and R'p as well as the input and output connections 11, 12, 16 and 17 are shown here.

The amplifier is described first. A first power takeover stage contains z \ e. Field effect transistors 18 and 19, whose cathode connections S are connected to a constant current source and whose anode connections are connected to two loads 21 and 22 are. This power takeover stage reinforces the connections 11 and 12 supplied signals and leads them to a second current transfer stage, which the transistors 23 and 24, a constant current generator 25 which is the resistors

and R ¹ . Two more with resistors R. and R ¹ . Transistors 26 and 27 connected as cathode followers amplify the signal appearing at the collectors of transistors 23 and and feed it to connections 28 and. Since there is no reversal between the terminals 11 and 17 and between the terminals 12 and 16, the feedback resistors R ₂ and R ¹ «must each between the terminal 16 and the control terminal of the field effect transistor 18 and the terminal 17 and the control terminal of the Fittd effect transistor 19 are switched in order to let the amplifier act as an arithmetic amplifier.

The low-mode signal is applied to input terminals 11 and fed and practical like M, a Kotodenfolder without significant change in amplitude and phase position and size and polarity of the control connections of the PieId effect transistors 18 and 19 to their cathode terminals.

909804/0713

. "■■ '^;-' SAD

-9-

An isolating amplifier 3D with a gain factor of 1 also transmits the common-mode signal to the collectors of the transistors 23 and 24 with practically no significant change in amplitude and phase or size and polarity. which are formed by the collectors of transistors 23 and 24 * and the amplifier 30 serves as a bypass device since it is a bypass for the common mode signal with respect to a part of the differential amplifier. From there it reaches the connections 16 and 17 through the cathode follower effect of the transistors 26 and 27, again largely without changing the amplitude and phase or size and polarity. The isolation amplifier 30 enables the cathode connections of the field effect transistors 18 and 19 to be connected to the collectors of the transistors 23 and 24 without the high impedance of the constant current source 20 being shunted. The internal resistance of the current source 20 should be as high-resistance as possible, so that practically 100 % of the common-mode signal is fed to the isolation amplifier 30.

The glj / real active suppression in the amplifier according to FIG. 2 can be improved even further if the influence of any imbalance in the control electrode-anode capacitance the field effect transistors 18 and 19, the influence any asymmetry in the output resistances and the capacities of their control connections to earth or Mass can be reduced. The current flowing through these capacitances as a result of the common-mode signal can flow through

909884/0713

Application of the common mode signal to the anode connections these field-effect transients can be reduced. For example, an amplifier with a gain factor between the connected anode connections of the field effect transistors 18 and 19, and the connected base connections of the two further transistors are designed, wherein the anode connection of each field effect transistor the emitter / collector path of one of the further transistors contains. In this way, what is already present at the cathode connections of the field effect transistors is removed Common-mode suitably supplied to their anode connectors.

Since the common mode rejection in the amplifier according to FIG. 2 and the mentioned modification of this amplifier depends on the fact that the compensation signal is largely equal to the common-mode signal, can be used as an amplifier with a gain factor 1 cathode follower can be used.

The cathode follower effect of the field-effect transistors 18 unu 19 v / ird by adding more transistors in their anode circuits further improved.

The amplifiers with gain factor 1 can be in .series * can be switched as shown in FIG. An amplifier with a gain factor of 1 connects here Field-effect transistor 32, the cathodes of the field-effect transistors 18 and. ^ 9 with the base connections of two transistors 33 and 34, which with the anode connections the field effect transistors 18 and 19 are connected.

909884/0713

BATHROOM 0RI3INAI:

The one at the cathode connection of the field effect transistor 32 appearing common mode signal is forwarded to amplifier 30, which is indicated by a dashed line and includes two transistors.

The output s signal of the amplifier 30, the wid "already mentioned, has a gain factor of 1, is then used as a compensation signal to the collectors of the transistors 23 and 24 Supplied via a Zener diode 35.

Another amplifier stage between the transistors 33 and 34 and the transistors 23 and 24 is through transistors 36 and 37 are formed.

Those components in FIG. 3 _f which have the same functions as the components according to FIG. 2 are provided with the same reference symbols. The function and mode of operation of the other components according to FIG. 3 are familiar to the person skilled in the art.

A capacitive imbalance in the amplifier's input circuit 3 can also be reduced by setting a differential capacitor 38 between the collectors of transistors 33 and 34 is located.

Although in those described with reference to Figs ExecutionExamples of solid-state components with a Cathode, an anode and a control connection are used circuit arrangements can also be built in which thermionic vacuum tubes are used, the all a cathode, an anode and a control connection to have.

909884/0713

Claims (1)

Claims I / differential amplifier circuit in which a differential amplifier with two input connections and two output connections is included, thereby marked ei without t that a diversion amplifier (30) is provided which is designed and arranged such that he supplies equalization signals to the output connections (28, 29) which are largely identical to common-mode signals, if * these occur in a predetermined part of the differential amplifier. 2) Circuit according to claim 1, characterized in that that the differential amplifier is a first Current transfer stage with active components (18, 19) »which are connected to the input terminals (11 and 12) and includes a second current take-over stage, the bypass amplifier (30) being between the common branch (20), the first power takeover stage and the output connections the active components (26 and 24) of the second current transfer stage lies. ψ 3) Circuit according to claim 2, d ... a by gek e η n draws that two further active components (23 and 34) are each connected to the anodes of the active components (18 and 19) of the first current transfer stage and from a further amplifier (32) can be controlled, which is connected to the common ■ branch (20) of the first current transfer stage » 909884/0713 BATH 4) Circuit according to Claim 2 or 3, characterized gekennaeic h η e t that isolation amplifier stages (36 and 37t 26 and 27) between the two current transfer stages and are connected between the second current transfer stage and the output connections (28 and 29). 5) Circuit according to one of claims 1 to 4 »d a du r c h characterized in that feedback impedances the Input connections (11 and 12) with the output connections (28 and 29) connect, 6) Circuit according to one of claims 1 to 5, as through ge characterized net that the active components Are solid-state components. 90.9034 / 07