JPH0389608A - High input impedance amplifier - Google Patents

Abstract

PURPOSE:To realize a high input impedance amplifier with less sensitivity variance and signal loss in comparison with a conventional amplifier using an FET by constituting the amplifier, which amplifiers the output signal of a condenser microphone, of only bipolar transistors TRs and a resistor. CONSTITUTION:A bias part 5 essentially consists of an operational amplifier A1, an amplifier A2 (the gain is, for example, one), TRs X1 and X2, and resistors R1 and R2, and amplifiers A1 and A2 form a negative feedback loop. An amplifier 6 consists of an amplifier A3 having the same constitution as the amplifier A2, and a condenser microphone 1 is connected to the connection point between the bias part 5 and the amplifier 6. The feedback loop consists of the operational amplifier A1, the amplifier A2, and the TR X1 and the loop controls to equalize two input voltages of the operational amplifier A1 and the TR X2 and the input of the amplifier A3 are connected so that the input voltage of the operational amplifier A1 is the bias voltage of a signal source 1. In this case, constant current circuits Ic1 and Ic2 of amplifiers A2 and A3 are constituted with minute currents.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a high input impedance amplifier, and more particularly to a high input impedance amplifier that requires a small input capacitance to amplify the output signal of a condenser microphone.

[Conventional technology]

Currently, condenser microphones are used in many fields for various purposes. Generally, the output impedance of a condenser microphone is as small as 10 pF to 20 pF, and therefore, in order to provide a microphone function, it is necessary to perform impedance conversion using a high input impedance amplifier with a small input capacitance.

FIG. 4 shows a circuit diagram of a conventional example. In the figure, an audio signal from a capacitor microphone 1 (audio signal source 1a and capacitive element 16) is transmitted through an FET 2a and a diode 2b.
The signal is amplified by a high input impedance amplifier 2 composed of , and outputted from an output terminal 3 . In this case, since FETs generally have extremely high input impedance, they are often used as amplifiers for condenser microphones.

[Problem to be solved by the invention]

FETG is often used as an amplifier for condenser microphones due to its high input impedance.
ETs generally have large variations in their characteristics, and therefore have the problem of large variations in bias current and bias voltage, resulting in large variations in sensitivity and signal loss.

An object of the present invention is to provide a high-performance amplifier with less variation in sensitivity and less signal loss when used as an amplifier for a condenser microphone.

[Means to solve the problem]

As shown in FIG. 2, the present invention is composed of only an operational amplifier AI which receives an arbitrary voltage as one input, a bipolar transistor and a resistor, and is provided with a constant current source 1o+ set to a minute current. The first amplifier A2 operates in a low current region using constant current characteristics at the output of the operational amplifier A1, and is composed only of a bipolar transistor and a resistor, and a first amplifier A2 that feeds back the output voltage to the other input of the operational amplifier A1. The circuit is equipped with a constant current source 102 set to a minute current, operates in the low current region at the output of the operational amplifier A1 and utilizes constant current characteristics, and is supplied with the signal source 1 to the input terminal. The base is commonly connected to the output terminal of the second amplifier A3, which amplifies and takes out the signal, and the operational amplifier A+.
The output of operational amplifier AI turns on and the first and second amplifiers A2 . The first and second transistors X+ conduct the respective input currents of A3 to operate them respectively.

The configuration consists of X2.

[Effect]

operational amplifier A+, first amplifier A2. The feedback loop composed of the first transistor X1 controls the two input voltages of 1 to the operational amplifier to be equal, and the connection between the second transistor x2 and the input of the second amplifier A3 creates an oil amplifier The input voltage of A1 is made to be the bias voltage of signal source 1. In this case, since the constant current circuit of the second amplifier A3 is configured to have a minute current of %i, it is possible to configure an amplifier with high input impedance and small input capacity using only bipolar transistors. Bipolar transistors generally have less variation in characteristics than FETs, so F
Compared to conventional high input impedance amplifiers using ET, there is no variation in sensitivity, signal loss can be reduced, and a high-performance amplifier can be obtained.

〔Example〕

FIG. 1 shows a circuit diagram of an embodiment of the present invention, and FIG. 2 shows a schematic circuit diagram thereof. In each figure, the same components are denoted by the same numbers and symbols. In the figure, 5 is a bias section, which mainly includes an operational amplifier AI, an amplifier A2 (gain is 1, for example), and transistors X+ and X2.

It consists of resistors R+ and R2, and an amplifier At.

A2 forms a negative feedback loop. Reference numeral 6 denotes an amplifier section, which is composed of an amplifier A3 having exactly the same configuration as the amplifier A2. A condenser microphone 1 is connected to a connection point between a bias section 5 and an amplification section 6.

Now, amplifiers A+ and A2 constitute a negative feedback loop, and the input voltage to transistor Q+ of amplifier A+ is
, the input terminal to the transistor Q2 is vl, this negative feedback loop performs IIJI so that the voltages V+ and V2 (=V<) are equal.

That is, transistor x1 is turned on by the output of amplifier A1, and the base current i of transistor Q3 of amplifier A2 is
es flows, and the base voltage of transistor Q4, which is configured as a pair with transistor Q3 (output voltage of amplifier A2)
1liil[ is applied so that V4 (-V2) is equal to the input voltage 1 of the amplifier A1. As a result, amplifiers A+ and A2 are in a balanced state, and V+ = V2 =
V3=V4. In this case, if the constant current 1 cc of the constant current circuit rot is made a minute current, the base current ie3 of the transistor Q3 will be a low current on the order of 1001)A to several nA.

Further, when transistor x1 is turned on, transistor x2 is also turned on, and the base current iss of transistor Q5 of amplifier A3 flows. In this case, if amplifiers A2 and A3, transistors x1 and x2, and resistors R1 and R2 are made the same, the base voltage of transistor Q5 of amplifier A3
) is the same as the base voltage ■3 of the transistor Q3 of the amplifier A2, and if the voltage at the output terminal 3 is v6, then V+''
=Vs”=Va. Constant current circuit Io+ of amplifier A2
Similarly, the constant current I of the constant current circuit I02 of the amplifier A3
If CC2 is set to a minute current, the base current ias of the transistor Qs will be a low current on the order of 10 (IIA to several nA), similar to the base current is3 of the transistor Q3 of the amplifier A2. The current flowing at the end is in the low current region and is flowing using the constant current characteristics of the transistor, so it is easy to ensure the input impedance of amplifier A3 to be 1000 MΩ or more, and the input capacitance is Can be set below F. That is, F
It is possible to configure an amplifier with high input impedance and small input capacity using only bipolar transistors without using ET, and it is possible to amplify the audio signal of the condenser microphone 1 that enters the base of the transistor Q5 and output it to the output terminal 3 in the same phase. .

The circuit of the present invention balances the input and output of amplifier A1 with a negative feedback loop in amplifiers A+ and A2, so that V+ -Vz
=V3 =V4, and set the bias voltage Vs of the capacitor microho>.1 so that it is equal to the voltage 1 in the amplifier A3, which has exactly the same configuration as the amplifier N A 2. Moreover, as mentioned above, by making the 1oz constant current circuit a minute current, it is possible to use only bipolar transistors or ICs.
In this case, an amplifier with high input impedance and small input capacitance is constructed without using any external elements such as capacitors. In this way, since the amplifier is configured with only bipolar transistors and resistors, there is no variation in sensitivity and there is less signal loss compared to an amplifier using FETs. In this case, if amplifier A2 is not used and amplifier A1
If a loop is constructed in which negative feedback is applied from the base of transistor Q6 using only A3 and A3, the input impedance cannot be made high because the audio signal will be negatively fed back, and such a construction is impossible.

FIG. 3 shows a circuit diagram of a main part of another embodiment of the present invention, in which the same components as in FIGS. 1 and 2 are given the same reference numerals. In FIG. 3, a bias current 1a3 is applied to the base of transistor Q3 of amplifier A2 by transistor Q3'.
0 is added, and the bias current 1B5111 by the transistor Qs' is applied to the base of the transistor Q5 of the amplifier A3'.
You may also add In this case, the operation and effects are the same as those of the embodiment shown in FIGS. 1 and 2, so the explanation thereof will be omitted.

It goes without saying that in each of the above embodiments, the same characteristics can be obtained even if the polarities of the illustrated transistors are reversed.

Further, the gain of the amplifier A2 is not limited to 1 as in the above embodiment, but may be set to be greater than 1.

〔Effect of the invention〕

According to the present invention, since it can be configured with only bipolar transistors and resistors, it is possible to realize a high input impedance amplifier with less sensitivity variation and signal loss than conventional high input impedance amplifiers using FETs, and also uses a capacitor. Since it is not done, it is easy to convert to tC.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of one embodiment of the present invention, FIG. 2 is a schematic circuit diagram thereof, and FIG. 3 is a circuit diagram of a main part of another embodiment of the present invention.
FIG. 4 is a circuit diagram of a conventional example. 1... Condenser microphone, 3... Output terminal,
5... Bias section, 6... Amplification section, VCC... Power supply, A1... Operational amplifier, A2. A3...Amplifier,
Q1~Qs, X+, X2- transistors, lot. 1o 2 ... constant current source.

Claims (1)

[Claims] It is composed only of an operational amplifier that receives an arbitrary voltage as one input, a bipolar transistor, and a resistor, and is provided with a constant current source set to a minute current, and the output of the operational amplifier is Operates in the low current region and utilizes constant current characteristics,
a first feeding the output voltage back to the other input of the operational amplifier;
It is composed only of an amplifier, a bipolar transistor, and a resistor, and is equipped with a constant current source set to a minute current, and operates in a low current region using the constant current characteristics at the output of the operational amplifier, A second amplifier is supplied with a signal source to its input terminal and amplifies and extracts the signal, and a base is commonly connected to the output terminal of the operational amplifier, and is turned on by the output of the operational amplifier to output the signal from the first and second amplifiers. A feedback loop consisting of the operational amplifier, the first amplifier, and the first transistor operates the operational amplifier. Controlling two input voltages of the amplifier to be equal, and connecting the second transistor to the input of the second amplifier so that the input voltage of the operational amplifier becomes the bias voltage of the signal source. A high input impedance amplifier characterized by the following configuration.