JPS635289Y2 - - Google Patents

Description

[Detailed description of the invention] The invention suppresses the reduction in the common-mode voltage discrimination ratio due to variations in the characteristics of a differential amplifier consisting of a pair of ordinary current-controlled transistors or FETs (field effect transistors), and the individual transistors. The present invention relates to a biological signal amplifier with a high input impedance and a high discrimination ratio.

This type of amplifier is well known, as shown in FIG. 1, which is equipped with a circuit for suppressing a reduction in the discrimination ratio caused by variations in the gm of FETs constituting a differential amplifier. That is, in FIG. 1, C1 and C2
is a pair of coupling capacitors for input, ZD
1 to ZD4 are Zener diodes for overvoltage protection,
R1 and R2 are a pair of series resistors for protection, C
3 and C4 are a pair of filter capacitors for high frequencies, and R3 and R4 are a pair of input resistors of the differential amplifier. A1 is a differential amplifier consisting of a pair of FETQ1, Q1', R5 and R6 are FETQ1, Q
A pair of resistors for source followers for each of FETs Q1', R7 and R8 are a pair of resistors for loads for each of FETs Q1 and Q1', and Q2 is a current feedback for keeping the sum of the drain currents of differential amplifier A1 constant. R9 functions as a drain current addition resistor for controlling the transistor Q2 in accordance with the sum current and a resistor for the base of the transistor Q2, and R10 functions as a feedback for the differential amplifier A1 and a load for the transistor Q2. resistance to,
A2 is an operational amplifier that amplifies the differential output of differential amplifier A1, R11 is a resistor for negative feedback to the source of one FETQ1, and R12 and VR1 are resistors and feedback for negative feedback to the source of the other FETQ1', respectively. A variable resistor R13 for adjusting the amount is a resistor for the output of the operational amplifier A2.

Normally, the resistors R1 and R2 are, for example, several kΩ, and R3,
R4 is on the order of 100MΩ and is connected to the reference potential E and +
The biosignal and the common mode voltage (HAM) between the and - input terminals are slightly divided, and the common mode voltage is largely canceled out in the differential amplifier A1.The biosignal is detected as a differential voltage and is amplified and output by the operational amplifier A2. be done. At this time, as shown in an exaggerated manner in FIG. 2, if there are variations in the drain current I _D with respect to the gate-source voltage V _GS of both FETs Q1 and Q1' of the differential amplifier A1, that is, the gm characteristic, the operating point should be set in advance to Even when set to the matching reference operating point A, the common mode voltage
If ei is input and V _GS correspondingly changes by △ei for both FETs Q1 and Q1′, then
Both _IDs tend to increase disproportionately. However, the sum of these currents controls the base of transistor Q2, increasing the voltage drop across resistor R10, and as a result, constant current control is performed to pull up the source voltages of both FETs Q1 and Q1' and return the operating point to reference point A. This suppresses a decrease in the discrimination ratio. However, in this current feedback circuit, both FETQ1 and Q
Even if the sum current of 1′ can be controlled to be almost constant,
FETQ1, Q corresponding to the input level of common mode voltage
Imbalance of drain current due to change in drain-source voltage V _DS of FET1', that is, both FETQ1, Q1'
The decrease in the discrimination ratio due to variations in internal resistance could not be suppressed. There was also the problem of a reduction in the discrimination ratio due to variations in the voltage division ratio, ie, the input levels to both FETs Q1 and Q1', due to variations in the constants of circuit elements R1 to R4, ZD1 to ZD4, etc. in the input circuit.

Therefore, it is an object of the present invention to provide a biological signal amplifier of the type mentioned at the beginning, which can further improve the common-mode voltage discrimination ratio.

Next, the present invention will be explained based on the illustrated embodiments.

FIG. 3 shows a circuit according to an embodiment of the invention, which has a Zener voltage corresponding to the operating voltage between resistors R7, R8 and R5, R6 in the reference operating state, with respect to the circuit according to FIG. 1; And both FETQ
1, a resistor R is used to average the source voltage of Q1' and return it to the drain side via resistors R5 and R6.
10 and a Zener diode ZD5 inserted between the collector of the transistor Q2, and a transistor whose base is connected to the cathode of the Zener diode ZD5, whose emitter applies the cathode voltage to the resistors R7 and R8, and whose collector current controls the transistor Q2. Q3, a resistor R14 that drives the breakdown of the Zener diode ZD5,
A bypass capacitor C5 for preventing oscillation, and a negative feedback circuit for increasing the input impedance, each composed of a capacitor C6 and a resistor R15, and a capacitor C7 and a resistor R16, are added.

The operation for common mode voltage input is as follows.

With respect to the common mode voltage input to the + input terminal and the - input terminal, the source outputs of both FETs Q1 and Q1' are negatively fed back through capacitors C6 and C7, respectively, with a feedback rate of approximately 1, and the input impedance is The voltage dividing elements C1 to C4, R1, R2, ZD1 to
The influence of impedance variations of 4 is correspondingly smaller. This improves the common mode voltage discrimination ratio by at least 10 dB compared to the input circuit shown in FIG.

Furthermore, in the differential amplifier A1, the amplitude is
When the common mode voltage is input to the gates of both FETs Q1 and Q1', the respective source voltages follow
increases by ei and attempts to decrease V _DS by ei.
At that time, as shown in an exaggerated manner in Figure 4, the V _DS −I _D characteristics,
That is, if there is variation in Rd, an imbalance will occur in the drain currents between both FETs Q1 and Q1' even though the sum of the drain currents is controlled to be constant by the transistor Q2. However, correspondingly, the voltage at the anode of the Zener diode ZD5 also changes due to the voltage change across both FETs Q1 and Q1'.
and R6, the drain voltage of both FETs Q1 and Q1' also rises by approximately ei due to the voltage feedback via transistor Q3, so that the operating point reaches the reference point in FIG. 4. It is controlled in the direction of pulling back to A. As a result, in addition to the conventional constant current control, the voltage feedback control according to the present invention is performed, so that both _FETQ1 , The deviation of Q1' _ID from the matching reference operating point is suppressed. As a result, it was confirmed that in addition to the increase in the discrimination ratio due to the input circuit improvement described above, it is possible to further improve the discrimination ratio by 10 dB or more.

For biological signals, the difference between the input levels to the gates of both FETs Q1 and Q1' is amplified by the differential amplifier A1 and the operational amplifier A2, and output from the resistor R13.
The differential amplifiers A4, ZD5, and Q3 have no effect on the differential amplification of the biosignal.
1. The possibility of high frequency oscillation occurring due to the closed loop through the transistors Q3 and Q2 and the Zener diode ZD5 can be effectively prevented by inserting a high frequency bypass capacitor C5 between the base and collector of the transistor Q2, which has a particularly large phase lag.

Note that this invention uses high h _FE instead of FETQ1, Q1'.
The present invention can also be applied to the case of using a transistor having a Darlington connection or a Darlington-connected transistor. Furthermore, the present invention is also applicable to P-channel type FET or PNP type transistors, in which case transistors Q2 and Q3 are also of opposite type.

As described above, according to the present invention, in addition to making the sum of the drain or collector currents of the transistors forming a pair of differential amplifiers a constant current, the respective drain-source or collector-emitter voltages can be made constant with respect to the common mode voltage input. This makes it possible to significantly improve the common-mode voltage discrimination ratio.

[Brief explanation of the drawing]

Figure 1 shows an example of the circuit configuration of a conventional biological signal amplifier.
Figure 2 shows the FETs that make up the differential amplifier.
Fig. 3 shows an example of the circuit configuration of the biological signal amplifier according to the present invention, and Fig. 4 shows the drain current characteristics with respect to the gate-drain voltage of the above-mentioned FET.
The drain current characteristics with respect to the source voltage are shown. A1...differential amplifier, A2...operational amplifier.

Claims (1)

[Scope of utility model registration request] A differential amplifier A1 consisting of a pair of transistors Q1 and Q1' each receiving a biological signal as an input;
The sources or emitters of the pair of transistors Q1 and Q1' are connected to one end of a pair of resistors R5 and R6, respectively, and the operational amplifier A2 and the reference potential E are connected to the sources or emitters of the pair of transistors Q1 and Q1'. Resistors R11 and R1 for negative feedback
2. A Zener diode with which VR1 is connected, a common feedback resistor R10 is connected to the other end of the pair of resistors R5 and R6, an anode is connected to this connection point, and a breakdown drive resistor R14 is connected to the cathode. ZD5 and a pair of transistors Q1,
A voltage feedback transistor Q3 whose emitters are connected to the other ends of a pair of resistors R7 and R8, one end of which is connected to the drain or collector of Q1', and whose base is connected to the cathode of a Zener diode ZD5.
The base current is controlled by the collector current of this transistor, and the collector is a Zener diode.
A biological signal amplifier characterized by comprising a transistor Q2 for current feedback connected to the cathode of the ZD5.