JP2002164749A - Impedance conversion amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an impedance conversion amplifier for receiving a signal with a high input impedance, and outputting a signal with a low output impedance, capable of obtaining the fixed degree of amplification by switching a switchgear while maintaining the functions. SOLUTION: This impedance conversion type amplifier is provided with a resistance 2 for deciding an input impedance, an FET 4 for receiving a signal, a transistor 6 to be turned ON/OFF by an interlocking switchgear 19, a transistor 10 for allowing the source current of the FET 4 to flow, a transistor 20 for outputting a signal, and an FET type arithmetic amplifier 15 constituting a feedback system to be operated so that a difference between a signal transmitted from the emitter side of the transistor 20 through a resistance 18 and a signal connected through a resistance 3 as the compared result of those signals can be offset. Thus, it is possible to reduce an output impedance according to the actions of the FET 4 and the transistor 6 and the transistor 20, and to obtain the fixed degree of amplification according to the rate of the resistance 17 and the resistance 18 to be connected by turning ON the interlocking switchgear 19.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high input impedance-low output impedance type impedance conversion amplifier.

[0002]

2. Description of the Related Art A conventional impedance conversion type amplifier shown in FIG. 5 receives a signal with a high input impedance.
A source follower composed of an FET 53 and its peripheral circuit, and an FET-type operational amplifier 54 are used. This F
The signal from the input terminal 51 is applied to the + (positive) terminal input of the ET type operational amplifier 54, and the emitter output voltage of the emitter follower 57 is applied to the-(negative) terminal. F
The ET type operational amplifier 54 operates so that the voltage difference between both terminals is eliminated, and is transmitted to a circuit group 55 including a common base type transistor 56. The grounded base transistor 56 constitutes a current source for the FET 53,
The output of the FET-type operational amplifier 54 is consequently converted into the collector current of the transistor 56 to determine the operating point of the FET 53.

In this conventional example, since a signal appears at the output terminal 61, the output impedance is determined by the collector resistor 62. In this conventional example, an AC component is transmitted to the input of the source follower type FET 53 by the coupling capacitor 52, and a DC component of the signal is reproduced by the FET type operational amplifier 54. Further, when the switch 60 connected to the emitter side of the emitter follower 57 is ON, the amplification degree is determined by the ratio between the resistance connected to the switch 60 and the collector resistance 62 of the emitter follower 57.

[0004]

However, a preamplifier using a conventional operational amplifier has a function of receiving a signal with a high input impedance and suppressing the emitter potential of the transistor to the same potential as the input potential. The output impedance is determined by the resistance connected to the collector, and a low output impedance cannot be obtained unless this resistance is kept low.On the other hand, in order to obtain amplification, the resistance on the emitter side must be reduced. There was a problem that had to be.

SUMMARY OF THE INVENTION The present invention has been made to solve such a conventional problem, and an impedance conversion type capable of obtaining a constant amplification degree and converting a high input impedance to a low output impedance. It is an object to provide an amplifier.

[0006]

According to the present invention, there is provided a source follower for receiving an input signal, a transistor constituting a constant current source of the source follower, and an emitter follower for outputting a signal.
In an impedance conversion amplifier having a gate input of a source follower and an emitter potential of an emitter follower as inputs and an output connected to a transistor constituting a current source, an FET input type operational amplifier, the source of the source follower and the transistor constituting the constant current source are connected to each other. A common-base transistor provided between collectors, switching means for switching the base potential of the common-base transistor, and FET
An impedance conversion type amplifier having a resistor for dividing an input from the emitter follower side of the input type operational amplifier and a selection means for selecting the resistance in conjunction with the switching means.

With this configuration, a certain degree of amplification can be obtained, and a high input impedance can be converted into a low output impedance.

Further, according to the present invention, a plurality of resistors are connected in series between an emitter of an emitter follower for outputting a signal and a ground potential, and a terminal capable of extracting a signal from a connection point between the resistors is provided. This is an impedance conversion type amplifier having a resistance type attenuator.

With this configuration, the input signal voltage can be attenuated to a voltage value determined by the resistance type attenuator.

[0010]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an impedance conversion type amplifier according to a first embodiment of the present invention. In FIG. 1, an impedance conversion type amplifier according to the present invention includes an input terminal 1 for applying a signal, a resistor 2 for determining an input impedance, and a resistor 3 for transmitting an input potential to a + terminal of an FET input type operational amplifier. An FET 4 having a gate connected to the input terminal 1;
One end is connected to the source of FET4, and the other end is a power supply (-
V), an NPN transistor 6 having a collector connected to the source of the FET 4, a base connected to the connection point of the resistors 7 and 8, and an emitter connected to the collector of the transistor 10 as a current source. , One end is connected to the base of the NPN transistor 20 and to the collector of the transistor 10, and the other end is connected to the power supply (+ V).
, And one end of the resistor 20 connected to the emitter and the output 38 of the transistor 20 and the other end connected to the power supply (-V) to operate the transistor 20 as an emitter follower.

In the impedance conversion amplifier according to the present invention, the base of the transistor 10 is connected to the resistors 11 and 12, and is fixed at a potential determined by both resistors. The emitter of the transistor 10 is connected to the output of the FET input operational amplifier 15 via the resistor 13 and to the power supply (-V) via the resistor 14.
A collector current substantially equal to the emitter current flowing through the resistor 14 and the resistor 13 flows through the transistor 10, forming a constant current source for the emitter of the transistor 6. FE
In the T-input type operational amplifier 15, a resistor 18 for feeding back the emitter potential of the transistor 20 is connected to a negative terminal, and the output thereof is connected to the resistor 13. Also,
The resistor 17 has one end connected to the negative terminal of the FET input type operational amplifier 15 and the other end connected to the interlock switch 19, and is grounded when the interlock switch 19 is selected to be turned on. Thus, the emitter potential of the transistor 20 is divided by the resistors 17 and 18.

The operation of the above-structured impedance conversion amplifier when the interlocking switch 19 is OFF (when OFF is selected) will be described with reference to FIG.

First, when a signal is input to the input terminal 1, F
A signal is transmitted to the gate of the source follower of ET4, and appears as a change in current on the source side. Since the base of the transistor 6 is connected to the power supply (+ V) via the resistor 7, the collector-emitter state becomes conductive, and the collector current (emitter current) is transmitted to the base of the transistor 20. It appears as a voltage signal on the emitter side. In this case, the amplification degree is about one time.

The FET input type operational amplifier 15 compares the input potential transmitted by the resistor 3 with the output potential fed back from the emitter of the transistor 20 of the emitter follower via the resistor 18 so that both outputs have the same potential. To the emitter of the transistor 10 via the resistor 13. As a result, the collector current of the transistor 10 is controlled, the drain current of the FET 4 changes through the transistor 6, and the gate-source voltage changes, thereby changing the gate potential of the FET 4 and the transistor 20.
Are operated so that the emitter potentials of the two become the same. In addition,
The variable resistor 16 is for adjusting the offset of the FET input type operational amplifier.

According to the configuration of the present invention, the input impedance as viewed from the input terminal 1 is determined by the resistor 2, but the forward DC current amplification factor h which is a parameter of the transistor 6.
fe1 and the forward DC current gain hfe2 which is a parameter of the transistor 20,
Output impedance Zou when viewed from the emitter side of
When t (output terminal 38) is simplified when the base impedance of the transistor 6 is Z1 and the base impedance of the transistor 20 is Z2,

[0017] As an example, hfe1 and
Assuming that hfe2 is about 100, the output impedance is 1 / 10,000 times the base-side impedance, which is a very small value.

Next, when the interlocking switch 19 is ON (ON
The operation will be described with reference to FIG. When the interlock switch 19 is ON, the resistor 17 is connected to the ground potential and the resistor 8 is connected to the power supply (-V). At this time, if the values of the resistors 7 and 8 are selected so that the base potential of the transistor 6 becomes negative, the transistor 6 is cut off, and the collector-emitter becomes non-conductive. Therefore, the source current of the FET 4 is supplied to the power supply (−
V), and the collector current of the transistor 10 constituting the constant current source flows to the resistor 9 from the power supply (+ V). Therefore, the resistor 9 is connected to the transistor 10
It operates as an apparent load resistance.

The voltage applied to the input terminal 1 is transmitted to the + terminal of the FET input type operational amplifier 15 via the resistor 3, and the voltage VE appearing at the emitter of the transistor 20 is supplied to the − (negative) terminal of the resistor 18 ( Rf) and the resistor 17 (Rs) are applied as follows.

The FET input type operational amplifier 15 outputs a voltage such that the difference between the voltages at both input terminals disappears due to its characteristics, and a voltage appears at the resistor 9 (R 9) via the resistor 13 and the transistor 10. The emitter voltage of the transistor 20 and the voltage of the input terminal 1 are controlled to be equal.

Therefore, when the interlocking switch 19 is ON, the amplification degree G is

As a result, it becomes possible to obtain a G-fold voltage.

The output impedance on the emitter side of the transistor 20 in this case is approximately

## EQU1 ##

Next, FIG. 4 shows an impedance conversion type amplifier according to a second embodiment of the present invention. 4, the resistors 32, 33, 34, and 35 are connected in series, one end of the resistor 32 is connected to the emitter of the transistor 20, and the other end of the resistor 35 is connected to the ground potential. The output terminal 38 is connected to the emitter of the transistor 20 via the resistor 31 and to the resistor 32. The output terminal 39 is at the connection point between the resistors 32 and 33, the output terminal 40 is at the connection point between the resistors 33 and 34 via the resistor 36, and the output terminal 41 is the connection point between the resistors 34 and 35 via the resistor 37. Connected to each other. The resistor 31 is connected to the emitter of the transistor 20 and determines the output impedance. The resistors 36 and 37 are output impedance adjusting resistors connected to be equal to the value of the resistor 31. As a result, the respective output impedances viewed from the respective output terminals 38, 39, 40, 41 have the same value.

As described above, according to the second embodiment of the present invention, each of the output terminals 38, 39, 40, and 41 can be selected by a switch or the like, and the resistors 31, 36, By providing the resistor 37, an output impedance is always kept constant with respect to an amplifier or the like connected to the next stage, and an effect of preventing a difference in frequency characteristics or the like from occurring regardless of which output terminal is selected by the switch. is there.

In the impedance conversion type amplifier shown in FIG. 4, resistors 32 to 35 are connected in series, and output terminals 39 to 41 are provided at the connection points of the resistors 32 to 35, respectively. Attenuator). In an actual example, the resistance 31 is set to 100Ω and the resistance 32
Is 200Ω, the resistance 33 is 100Ω, the resistance 34 is 60Ω,
When the resistance 35 is 40Ω, the resistance 36 is 25Ω, and the resistance 37 is 64Ω, the output impedance of each of the output terminals 38 to 41 is 100Ω in each case. Output terminal 38
Is 1, the output terminal 39 is １ ／
A voltage of 1/4 can be obtained at the output terminal 40 and a voltage of 1/10 can be obtained at the output terminal 41. If the voltage appearing at the output terminal 38 is insufficient, turning on the interlocking switch 19 in FIG.

[0028]

As described above, the present invention provides a source follower for receiving an input signal, a transistor constituting a constant current source of the source follower, an emitter follower for outputting a signal, a gate potential of the source follower and an emitter potential of the emitter follower. In an impedance conversion type amplifier having an FET input type operational amplifier in which an input is connected and an output is connected to a transistor constituting a current source, a grounded base type provided between a source of a source follower and a collector of a transistor constituting a constant current source A transistor, switching means for switching a base potential of a common-base transistor,
By having a resistor for dividing the input from the emitter follower side of the FET input type operational amplifier and a selecting means for selecting the resistor in conjunction with the switching means, it is possible to obtain a constant amplification degree and to reduce the high input impedance. An object of the present invention is to provide an impedance conversion type amplifier capable of converting the output impedance.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an impedance conversion type amplifier according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram of the operation when the interlocking switch of the impedance conversion amplifier according to the first embodiment of the present invention is OFF.

FIG. 3 is an explanatory diagram of the operation when the interlocking switch of the impedance conversion amplifier according to the first embodiment of the present invention is ON.

FIG. 4 is a circuit diagram of an impedance conversion type amplifier according to a second embodiment of the present invention.

FIG. 5 is a circuit diagram of a conventional impedance conversion type amplifier.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 input terminal 2, 3 resistor 4 FET (source follower) 5 resistor 6 NPN transistor 7 to 9 resistor 10 NPN transistor 11 to 14 resistor 15 FET input operational amplifier 16 variable resistor 17 resistor (Rs) 18 resistor (Rf) 19 Interlock switch 20 NPN transistor 21 Resistance 31-37 Resistance 38-41 Output terminal

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5J090 AA01 AA45 CA74 CA88 DN02 FA18 HA02 HA09 HA18 HA25 HA26 HA29 HA40 HN07 KA01 KA17 KA23 MA01 MA02 MA04 MA11 MA24 MN01 NN06 5J091 AA01 AA45 CA74 CA88 FA18 HA02 HA09 HA26 HA25 HA26 HA26 KA01 KA17 KA23 MA01 MA02 MA04 MA11 MA24 5J098 AA02 AA03 AA11 AB02 AB34 AC05 AC20 AD01 GA01

Claims (2)

[Claims] 1. A source follower for receiving an input signal, a transistor constituting a constant current source of the source follower, an emitter follower for outputting a signal, a gate potential of the source follower and an emitter potential of the emitter follower as inputs, and the output is the current An impedance conversion type amplifier having an FET input type operational amplifier connected to a transistor forming a source, a grounded base transistor provided between a source of the source follower and a collector of a transistor forming the constant current source; Switching means for switching the base potential of a common-base transistor; a resistor for dividing an input from the emitter follower side of the FET input operational amplifier;
An impedance conversion type amplifier comprising a selection means for selecting the resistor in conjunction with the switching means. 2. A resistor type wherein a plurality of resistors are connected in series between an emitter of an emitter follower for outputting the signal and a ground potential, and a terminal capable of extracting a signal from a connection point between the resistors is provided. The impedance conversion type amplifier according to claim 1, further comprising an attenuator.