JPS58111415A - Offset cancelling circuit for differential amplifier - Google Patents

Abstract

PURPOSE:To prevent the operating point from being fixed and the operating range from being narrowered, by feeding-back a DC output of an amplifier negatively to an input side and zeroing a DC offset automatically. CONSTITUTION:DC currents I1i, I2i being equal normally are applied to input terminal IN1, IN2 of a differential amplifier 1 via impedance ZA, ZB from a bias voltage VCT, and when ZA, ZB are equal with each other, potentials at base connecting points A, B, collector connecting points C, D, and output side connecting points E, F of transistors (TR) 1, 2 are equal and a DC offset is zero. While ZA>ZB, the base potential of the TR1 is lower than that of the TR2. This potential difference is amplified and applied to an offset detector 21 of a differential amplifier 2 as a DC offset voltage. As a result, the amplifier 2 feeds back the DC output negatively to the input side and the base potential of the TR1 is made higher than that of the TR2, allowing to zero the DC offset voltage automatically.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a DC auto-set canceling circuit 1, particularly to a circuit that automatically cancels a DC offset of an output signal from a differential amplifier.

(2) Background of the technology As the magnetic disk #10 head, a 1-magnetic resistive element whose impedance changes with changes in the magnetic field is capable of magnetoelectric conversion with infinite sensitivity.
It is beginning to be widely used. A constant bias 11 is applied to such an i-Dannet resistive element to determine the operating point of the signal, and the extracted signal is amplified by a differential amplifier and connected to the next stage circuit.

(3) Prior art and problems Figure 1 shows a general differential amplifier, and the human input signal FiXN
1. xN2! :*Increased amplification 0UT1゜0UT2
is output to. VCC is a positive power supply, VEIC is a negative power supply, and R1 and R2 are human termination resistances. differential amplification io
*The m stage is a transistor Ql.

Q2, resistors R5 and R4, current source 11, and amplifier-B.

In this case, conventional differential amplifiers may generate offsets (due to internal and external factors), causing fluctuations in the production point. In other words, two transistors Q, ,Q forming a differential pair
, or changes in the signal source connected to the input terminal IN□IN, for example, the imbalance of the two impedances that make up the 1st generation magnet resistive Giko MR is the main factor. , 1112 The human power voltage vX) I and the output voltage V of the differential amplifier shown in FIG. On the static characteristic curve of Vt, when there is no signal, the input terminal IN due to the nine factors mentioned above.
,.

When INjK DC-like differential voltage Δv1 is applied, an off-seven F voltage ΔV· is generated, and the operating point deviates from the original 0 point and moves to PAK. As a result, the static characteristic f
! The signal no longer fits within the linear portion AB on the 4th axis, and the output signal 8° becomes a waveform that is rejected with respect to the human input signal 8K, and only the portion shown by the broken line is not faithfully reproduced. In other words, the dynamic range becomes narrower.

As described above, in the conventional technology, the operating point fluctuates (DC component offset > K), and the output signal is not faithfully reproduced with respect to the input signal, resulting in a narrowing of the operating range.

+4) Purpose of A: The purpose of the present invention is to fix the operating point and narrow the operating range by negatively feedbacking the DC output of a differential amplifier to the negative input and automatically setting the DC offset to 0. The goal is to confront the enemy.

(5) Structure of the Invention According to the present invention, the differential output of a differential amplifier is human-powered, and the differential output has l# wavenumber characteristics sufficiently lower than the I# wavenumber of the human-powered signal of the differential amplifier. A differential amplifier comprising: an amplifier that amplifies a DC component that is lost and outputs it as an error signal; and a bias current source that changes a bias current applied to each input terminal of the differential amplifier according to the error signal. An amplifier offset cancellation circuit is provided.

(6) Embodiments of the Invention The present invention will be described below with reference to embodiments and the accompanying drawings.

Figure IN5 is a configuration diagram of an offset canceling circuit of a differential amplifier according to the present invention. The circuit of the present invention has a human power terminal IN1.
．． Magnetic resistive element M input from IN
The difference between each signal from R is amplified and the output terminal 0UTI, ou
Out of the output from the first differential amplifier m1iill and output terminal OUT outT, only the tl#11 portion is amplified and negatively fed back to the human input terminals IN, IN2. #I2 differential intensifier 2.

111 Differential increase 1110 Human power bite IN,, IN, to!
A Dunnett resistive element MR (hereinafter referred to as MR) is connected.

The above MR has an impedance ZAIZI, and as shown in the figure, a voltage VCT is applied to one tap, and a voltage tIL
122 K X D'ttL"t'tL Il 1.
Il i O (7 currents are supplied. Resistors R,, R
, does not need to be 4 times the ninth 40 of the amplifier bias, but its value should be sufficiently larger than 2 zIIK.

When MR and magnetic flux φ1 interlink in the direction shown in the figure, 2m increases, zl decreases, and 1! AC signal voltage Δ proportional to
V- is the transistor Tr1. Joins the base of Tr2.

Transistor fr1. The transistor forms a differential pair, and the above signal voltage ΔVs is applied from the resistors R, , R40 connection points C and D.
Amplify and take out nine voltages proportional to , and supply them to the amplifier AMP.

The 1112 differential amplifier 182 is connected to the output side connection points E and F of the 1111 differential amplifier 1, and the DC component VΣG, v, , am
The offset detection circuit 21 increases and extracts the Tr7.
Tr@4p: Consists of a bias current [22] that changes the I-rector current and changes the bias current flowing through the MR3g I/key dances zA and zll from the connection points A and B.

The offset detection circuit 21 is connected to the gate pressure Vc@ and the IIE Nagarashi 1.
2s) Tungista Try, Tr5 and Tr4. T
It has r4. Resistor R5Q R4 is the differential amplifier 111 of IN2 connected as a load of the differential amplifier 1 of +41
The i9 condenser C forms a low-pass filter together with the resistors a, R9, and is connected to the rounding that flows in from the connection points E and F to eliminate the alternating current component of the good signal. has been done.

Transistor Tr7. which constitutes bias current source 22. T
Negative power supply ■ for r6 pace. Nyota resistance R,,R,
. , a constant current is supplied through the diode D, and the resistors R, , R, convert the DC current supplied from the offset detector 21 into voltage, and the transistors T, , Tr
, has started to improvise on Niotsuku. Above Ty7
．． As mentioned above, Tr- appears at R.sup. and R9 and changes the collector current in proportion to the change in running voltage. Sunawa ZA, Z
There is a heat effect that changes the DC bias current tJt to l.

The operation of the circuit of the present invention configured as described above is as follows.

$11 differential increase! From the 11 input terminals IN, , IN, normally equal direct current 1. ,,I□ is the bias voltage v
, T is supplied to Z, , Z, and when Z, gZ, O, the output side contact/value point E is connected to the output side connection point C, DI! , F4D potentials are equal to each other, that is, the DC component offset is O.

However, the MROZOzlK imbalance is
, o, the base potential of Trl is lower than the base potential of Tr2. This potential difference is amplified and appears as an output me, FK DC off-foot voltage, which is supplied to the off-foot detector 21 of the stall increase@-2.

In this case, (Z,>Z,)V, the voltage of G is V,. The current of r4 is larger than the voltage of T, the bias current +][22 of resistors R,, R, has a current of T,
The potential of the emitter of r@ is higher than the potential of the emitter of 〒, 7, so as a result, T1. The collector current l current is Tr
7, p is smaller than that of Tr2, which is above the base potential of Trl.
It works to raise the base potential higher than the base potential of. That is, the currents of the inputs 111 and I21 automatically change so that the output OFF voltage becomes 0.

Also, even if MROZA=ZII, I, .

As I21 was unbalanced, transistor Tr1
．． When Tr2 is unbalanced and 9υ, R, and R4 are unbalanced, a DC offset voltage is generated at the output as described above. Even in this case, the differential amplifier 2 works so that the output offset voltage becomes O.
O current changes automatically.

Incidentally, when the amplifier is configured as a monolithic ICf, the capacitor C can be omitted by using a latex PNP transistor with low frequency characteristics as the transistor Tr3+'rr4 of the off-circuit detector 21.

In addition, in the embodiment shown in Figure 2, negative feedback is provided from the output circuit of the amplifier AMP, but if the gain of AMP is mostly 1, the output offset is determined by the offset of the previous stage of the amplifier AMP, so from the connection point CD Negative feedback may also be provided.

(7) Effects of the Invention According to the present invention, the DC output of the differential amplifier is negatively fed back to the input terminal, the bias current at the input terminal is changed, and the DC offset circuit F is automatically set to O. This makes it possible to eliminate the narrowness of the dynaic range.

[Brief explanation of drawings]

FIG. 11 is a conventional differential amplifier. FIG. 2 is a diagram showing the human output characteristics of the prior art. FIG. 3 is a block diagram showing the offset canceling path of the differential amplifier according to the present invention. 1... I11 differential amplifier, 2... Second differential amplifier,
21... DC offset detection circuit, 22... Bias current source. −8[

Claims (1)

[Claims] An increase aS that uses the O differential output of the differential amplifier as a human power, has a frequency characteristic sufficiently lower than the 1# wave number of the human power signal of the differential amplifier, and amplifies the DC component included in the differential output and outputs it as an error signal. #An offset canceling circuit for a differential amplifier, characterized in that it includes a bias current source that utilizes a bias current applied to each input terminal of a differential amplifier in response to an error signal.