JPH05121953A - Amplifier - Google Patents

Abstract

PURPOSE:To obtain the amplifier whose operation is stable against the dispersion in the characteristic of an amplifier element and a change in ambient temperature. CONSTITUTION:A voltage across a load resistor RC of a transistor(TR) Q1 is controlled to be equal to a reference voltage of a reference power supply VREF by a voltage control means. The reference voltage is changed to cancel a change in the gain of the TRs Q1, Q2 according to the ambient temperature. An AC negative feedback is applied from an emitter of the TR Q2 to a base of the TR Q1. A DC negative feedback is applied to a base of the TR Q1 by the voltage control means. A temperature coefficient is given to an AC negative feedback resistor RFac and a DC negative feedback resistor RFdc so as to cancel a change in an input impedance of the amplifier changing according to the ambient temperature.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an amplifier, and more particularly to an amplifier which has low noise and is stable against variations in the characteristics of the amplifying element and changes in ambient temperature.

[0002]

2. Description of the Related Art In the field of electronic measurement, a signal from a sensor is often weak, and it is required to amplify this signal with low noise and high stability. Therefore, in the amplifier used in the field of electronic measurement, the circuit configuration shown in FIG. 3 has been conventionally used.

The amplifier 11 shown in FIG. 3 has a two-stage configuration using two transistors Q1 and Q2 whose emitters are grounded.
The input signal is input to the base of the transistor Q1 of the first stage via the capacitor C1, and the output signal is taken out from the collector of the transistor Q2 of the second stage via the capacitor C2. In order to increase the stability of the operation, the first resistor from the emitter of the second-stage transistor Q2 is fed through the feedback resistor RF.
Negative feedback is applied to the base of the transistor Q1 of the stage.
RC is the collector resistance of the transistor Q1, RE is the emitter resistance of the transistor Q2, and R1 is the collector resistance of the transistor Q2.

Generally, in order to match the input impedance of the amplifier with the signal source resistance (for example, 50Ω), a matching resistor having a resistance value equal to the signal source resistance may be connected in parallel to the input terminal of the amplifier. However, this increases the noise because the matching resistor becomes a new noise source. Therefore, it is desired to match the input impedance without using the matching resistor.

The conventional amplifier 11 of FIG. 3 equivalently realizes the matching of the input impedance by negative feedback without using a matching resistor as a noise source. By doing so, it is not necessary to connect a resistor to the input terminal, and it is possible to perform amplification with low noise.

The collector resistance RC is, for example, 2.2 k.
Ω, the emitter resistance RE is 200Ω, the collector resistance R1 is 360Ω, and the feedback resistance RF is 2.2k, for example.
Ω, the capacitor C1 is, for example, 2000 pF, and the capacitor C2 is, for example, 10,000 pF.

[0007]

The conventional amplifier 11 described above is used.
Has a problem in that it is difficult to set the operating point because there are many parameters to be set, and the feedback resistor RF determines the operating point and the feedback amount, so that the setting range of the input impedance is limited.

Further, if the characteristics of the transistors Q1 and Q2 have variations, the variations are directly reflected in the operation, and it is difficult to obtain uniform amplification characteristics.

Further, due to the temperature dependence of the transistors Q1 and Q2, the gain and the input impedance also fluctuate when the ambient temperature fluctuates, which makes it difficult to perform highly stable amplification.

Therefore, an object of the present invention is to provide an amplifier whose operation is stable against variations in the characteristics of the amplifying element and changes in the ambient temperature.

Another object of the present invention is to provide an amplifier in which the operating point can be easily set and the degree of freedom in design is large.

[0012]

The first amplifier of the present invention controls the voltage across the amplifying means and the load of the amplifying means to be equal to the reference voltage, and the reference voltage is set to the ambient temperature. A voltage control means that changes so as to cancel the change in the gain of the amplification means, an AC negative feedback means that applies AC negative feedback to the amplification means, and a DC negative feedback means that applies DC negative feedback to the amplification means. It is characterized in that the AC negative feedback means and the DC negative feedback means have a temperature coefficient that cancels a change in the gain of the amplifier which changes according to the ambient temperature.

A second amplifier according to the present invention comprises a first amplifying means, a second amplifying means coupled to the first amplifying means, and both ends of at least one load of the first amplifying means and the second amplifying means. Voltage control means for controlling the voltage of the first amplifying means and the second amplifying means so as to cancel the change of the gain of the first amplifying means and the second amplifying means according to the ambient temperature. (2) AC negative feedback means for applying AC negative feedback from the amplifying means to the first amplifying means, and DC negative feedback means for applying DC negative feedback to the first amplifying means from the voltage controlling means. It is characterized in that the feedback means and the DC negative feedback means have a temperature coefficient that cancels the change in the gain of the amplifier which changes according to the ambient temperature.

[0014]

In the first amplifier of the present invention, since the voltage across the load of the amplifying means is controlled to be equal to the reference voltage, the bias voltage and the bias current of the amplifying means are stabilized, and as a result, the operation is performed. Is stable.

When the ambient temperature changes, the reference voltage of the voltage control means changes accordingly so as to cancel the change of the gain of the amplification means, and the AC negative feedback means and the DC negative feedback means change the gain of the amplifier. Since it changes so as to cancel the change, it is stable against changes in ambient temperature.

In the second amplifier of the present invention, the voltage across the load of at least one of the first amplifying means and the second amplifying means is controlled to be equal to the reference voltage. The bias current is stabilized, resulting in stable operation.

When the ambient temperature changes, the reference voltage of the voltage control means changes correspondingly so as to cancel the change of the gains of the first and second amplification means, and the AC negative feedback means and the DC negative feedback means are Since the change is made so as to cancel the change in the gain of the amplifier, the amplifier is stable even when the ambient temperature is changed.

In both the first and second amplifiers of the present invention, the AC feedback amount and the DC feedback amount can be set independently of each other, so that the operating point can be set easily and the degree of freedom in designing is increased. .. For example, it is possible to set DC negative feedback in the first stage at the point of minimum noise, and to set AC negative feedback independently so that the input impedances match.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present invention will be described below with reference to FIG.

FIG. 1 is a circuit diagram showing an embodiment of the amplifier of the present invention.

The amplifier 1 of the present invention has a grounded emitter 2
It has a two-stage configuration using individual transistors Q1 and Q2. Further, it has an error amplifier Q3 and an inverting amplifier Q4 for stabilizing the bias voltage supplied to the transistors Q1 and Q2.

Vcc is a bias power source, VREF is a reference power source for generating the reference voltage Vstd of the error amplifier Q3, DC bias power source, RC is a collector resistance of the transistor Q1, and RE is an emitter resistance of the transistor Q2.

The input signal is passed through the capacitor C1 to the first
It is input to the base of the transistor Q1 of the stage. The output signal is taken out from the emitter of the second stage transistor Q2. The operation of the transistors Q1 and Q2 is the same as the conventional amplifier 1
It is the same as that of 1.

In the amplifier 1 of the present invention, the conventional amplifier 1
Different from 1, the negative feedback is applied by dividing into direct current and alternating current. That is, AC negative feedback is applied to the base of the transistor Q1 of the first stage from the emitter of the transistor Q2 of the second stage via the AC feedback resistor RFac and the capacitor C4. Further, the output of the error amplifier Q3 is sent to the base of the transistor Q1 of the first stage through the inverting amplifier Q4, the DC feedback resistor RFdc and the resistor R3, so that the DC negative feedback is applied. The DC feedback circuit is grounded between the DC feedback resistor RFdc and the resistor R3 via the resistor R4 and the capacitor C5. The minus terminal of the error amplifier Q3 is connected to the collector of the transistor Q1 via the resistor R2, and the plus terminal thereof is connected to one end of the collector resistor RC of the transistor Q1 via the reference power source VREF.
A part of the output of the error amplifier Q3 is fed back to the-terminal via the capacitor C3.

The error amplifier Q3 is a differential amplifier, and when the voltage across the collector resistance RC fluctuates and an error occurs between the error and the reference voltage Vstd given by the reference power supply VREF, the error amplifier Q3 is amplified and amplified. Inject the correction signal into the base of Q1. In this way, the error amplifier Q3 detects the voltage across the collector resistance RC of the transistor Q1 and operates so that the voltage is always equal to the reference voltage Vstd.
As a result, assuming that the current flowing through the collector resistance RC becomes constant and the base current flowing into the transistor Q2 can be ignored, the current flowing through the transistor Q1 becomes constant. At this time, if the bias power supply Vcc is constant, the collector voltage of the transistor Q1 is also constant, and the gain of the first-stage amplifier is stable. The transistor Q2 is an emitter follower having a voltage gain of about 1 and the gain is basically stable.

The reference power source VREF is designed so that the reference voltage Vstd changes according to the change in ambient temperature, thereby canceling the change in the gain of the transistor Q1 caused by the change in ambient temperature. The reference power supply VREF having such a function can be realized by using a resistor whose resistance value changes at a desired rate of change in proportion to the ambient temperature. Alternatively, it can be realized by using a known IC capable of taking out a voltage or current proportional to the ambient temperature.

The inverting amplifier Q4 is connected to the output of the error amplifier Q3 to match the polarity.

Error amplifier Q3, inverting amplifier Q4, resistor R
2, R3, R4, DC feedback resistor RFdc, reference power supply VREF, and capacitors C3, C5 constitute voltage control means.

Since the amplification degree of the amplifier 1 of the present invention is determined by the physical constants of the transistors, it is basically constant regardless of variations and types of the transistors Q1 and Q2. Therefore, the reproducibility is excellent, the temperature can be corrected well, and the gain stability of, for example, 100 to 200 ppm / ° C is not difficult. Further, since the bias voltage and the bias current are kept constant and the negative feedback is applied, the amplifying action of the transistors Q1 and Q2 is also stabilized. Furthermore, since the matching of the input impedance is equivalently realized by the negative feedback without using the matching resistor as the noise source, the noise is low.

Further, in the amplifier 1 of the present invention, since the negative feedback circuit is divided into the AC feedback circuit and the DC feedback circuit, there is an effect that the amount of feedback can be easily set.

Next, the conditions under which the gains and input impedances of the transistors Q1 and Q2 are kept constant even if the ambient temperature changes will be described.

Considering one transistor, the voltage gain Gv in the case of grounded emitter is expressed as follows.

First, assuming that the feedback resistance of the transistor is Rc, the emitter current ie is: ie = VREF / Rc Further, the transconductance gm is gm = q.ie / kT where q is the electron charge and k is the Boltzmann. The constant T is an absolute temperature.

Therefore, the voltage gain Gv of the transistor is
Gv = gm.Rc = (q / kT) .VREF ........ (1)

From the equation (1), it can be seen that the voltage gain Gv changes in inverse proportion to the temperature T. Therefore, the reference voltage V
If REF is changed in proportion to the temperature T, the voltage gain Gv does not change with the temperature T.

That is, if the reference voltage VREF is changed so that (VREF / T) = α (constant), Gv = (q / k)  (VREF / T) = (q / k) α ... (2) The voltage gain Gv can be kept constant even if the ambient temperature changes.

Also, as will be described below, the input impedance Zi of the transistor also changes with changes in ambient temperature.

The input impedance Zi of the grounded-emitter transistor is Zi = rb + (hfe / gm) = rb + (q.alpha..hfe / k.Rc) where rb is the base resistance of the transistor and hfe is the current amplification factor. , Rc are feedback resistors.

In a high frequency transistor, rb is usually several Ω
Since it can be ignored, Zi ≈ (q · α / k · Rc) · hfe (3).

The current amplification factor hfe is usually +7000 ppm
Since the temperature coefficient is about / ° C, it can be seen that the input impedance Zi changes with the ambient temperature.

Therefore, in the amplifier 1 of the present invention, the AC feedback resistor RFac is set so as to satisfy the relation of the above equation (3).
If a temperature coefficient is given to the resistance value of, the temperature change of the input impedance of the amplifier 1 can be compensated, and as a result,
It is also possible to keep the input impedance and voltage gain of the amplifier 1 constant over a wide temperature range.

For example, in the case of an amplifier having a voltage gain of 40 dB and an input impedance of 50Ω, according to a simulation, if a temperature coefficient of −800 to −850 ppm / ° C is given to the feedback resistor, the change of the input impedance due to the temperature change can be canceled. The input impedance can also be made constant over a wide temperature range.

However, in practice, even if a resistance having a negative temperature coefficient is not used for the AC feedback resistance RFac, if the signal source resistance is constant, by giving a corresponding temperature coefficient to the reference voltage VREF, the finish is obtained. It is possible to have a constant voltage gain over a wide temperature range.

The collector resistance RC is, for example, 750.
Ω, emitter resistance RE is, for example, 1 kΩ, AC feedback resistance RF
ac is, for example, 11.5 kΩ, DC feedback resistor RFdc is, for example, 20 kΩ, resistor R2 is, for example, 1 MΩ, and resistor R3 is, for example, 1
kΩ, the resistance R4 is 1 kΩ, the capacitor C1 is 5 μF, the capacitor C3 is 0.33 μF, the capacitor C4 is 0.33 μF, and the capacitor C5 is 0.33 μF.

The voltage amplifying means does not have to have the above-mentioned structure but may have another structure as long as it operates in the same manner as described above. Further, as the amplifying element, transistors Q1 and Q2 are used.
Other known amplifying elements (for example, FET and HEMT) can be used.

In the above-described embodiment, the circuit configuration of two-stage amplification is used, but one-stage amplification or three or more stages may be used.

[0047]

As described above, the operation of the amplifier of the present invention is stable against variations in the characteristics of the amplification element and changes in the ambient temperature. Further, there is an effect that the operating point can be easily set and the degree of freedom in design is large.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of an amplifier according to the present invention.

FIG. 2 is a circuit diagram showing a schematic configuration of the amplifier of FIG.

FIG. 3 is a circuit diagram showing an example of a conventional amplifier.

[Explanation of symbols]

 1 amplifier Q1, Q2 transistor Q3 error amplifier Q4 inverting amplifier RC collector resistance RE emitter resistance RFac AC feedback resistance RFdc DC feedback resistance R2, R3, R4 resistance C1, C3, C4, C5 capacitors VREF reference power supply VCC bias voltage

Claims (2)

[Claims] 1. An amplifying means and a voltage across a load of the amplifying means are controlled to be equal to a reference voltage, and the reference voltage cancels a change in the gain of the amplifying means according to an ambient temperature. It comprises a voltage control means for changing, an AC negative feedback means for applying AC negative feedback to the amplifying means, and a DC negative feedback means for applying DC negative feedback to the amplifying means, and the AC negative feedback means and the DC negative feedback. An amplifier characterized in that the means has a temperature coefficient such that it cancels the change in the gain of the amplifier which changes according to the ambient temperature. 2. A first amplifying means, a second amplifying means coupled to the first amplifying means, and a voltage across at least one load of the first amplifying means and the second amplifying means being equal to a reference voltage. And a reference voltage that changes so as to cancel the change in the gain of the first amplifying means and the second amplifying means according to the ambient temperature, and the second amplifying means from the first amplifying means. It comprises an AC negative feedback means for applying AC negative feedback to the amplifying means, and a DC negative feedback means for applying DC negative feedback to the first amplifying means from the voltage control means, the AC negative feedback means and the DC negative feedback means. An amplifier having a temperature coefficient that cancels a change in the gain of the amplifier that changes according to the ambient temperature.