JPH02137510A - Current controlled gain variable amplifier - Google Patents

Abstract

PURPOSE:To attain low distortion factor and to expand an allowable input by devising a variable resistance element such that an NPN transistor(TR) and a PNP TR are connected in parallel, giving a control current to the NPN TR and obtaining the current from the PNP TR. CONSTITUTION:The variable resistive element of current control type in which NPN TRs 12, 13, PNP TRs 14, 15, an operational amplifier 11, a flowing control current source 16, a flowing-out control current source 17 and a fixed resistor 18 are connected as specified and the saturation region of the complementary TRs are utilized, and an inverse amplifier using the operational amplifier are constituted. In the variable resistive element, a control current is supplied to the base of the NPN TR 12 and the same quantity of the control current is obtained from the base of the PNP TR 14, then the input signal current and the control current are separated and the distortion factor due to superimposition is reduced, Moreover, the saturation region of the variable resistive element is spread over a range equal in both polarities by the parallel combination of the TRs of the both types and the permissible input is increased.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a current controlled variable gain amplifier used in a variable gain amplifier.

2. Description of the Related Art In recent years, current-controlled variable amplifiers have been used for noise reduction in cassette decks, high-fidelity VTRs, etc., but improvements in distortion rate and expansion of permissible input are desired.

An example of the conventional current-controlled variable gain amplifier mentioned above will be described below with reference to the drawings.

FIG. 3 shows a circuit of a conventional current controlled variable gain amplifier. In FIG. 3, 1 is an operational amplifier, 2,
3 is an NPN transistor, 4 is a constant current source with a value of 10
15 is a resistor whose value is RO16 is a signal source.

Regarding the current controlled variable gain amplifier configured above,
The operation will be explained below. First, a few NPs
A variable resistance element is constituted by N transistors, and between the emitter of the NPN transistor 2, the collector of the NPN transistor 2, and the emitter of the NPN transistor 3 operate as a resistance, and the current value of the constant current source 4 is . The gain Av can be changed by changing To.

Problems to be Solved by the Invention However, in the above configuration, the signal current and the control current flow simultaneously in the variable resistance element, and when the signal source is positive, the signal current flows from the emitter of the N P N l-transistor of 2. flows to the collector. If this signal current is large, the DC amplification factor in the reverse direction is small and the output voltage waveform is distorted, resulting in the problem that it is impossible to lower the distortion rate or expand the allowable input.

In view of the above-mentioned problems, the present invention provides a current-controlled variable gain amplifier that has a low distortion rate and an expanded allowable input.

Means for Solving the Problems In order to solve the above-mentioned problems, the current control type variable gain amplifier of the present invention has variable resistance elements in which NPN transistors and PNP transistors are connected in parallel, and control is performed. By flowing current into the NPN transistor and flowing it out from the PNP transistor, it has a configuration of an inverting amplifier using a low distortion, large input variable resistance element and an operational amplifier.

Operation The current controlled variable gain amplifier of the present invention achieves low distortion by separating the signal current and control current in the variable resistance element with the above configuration.

Further, by combining an NPN transistor and a PNP transistor with a variable resistance element, a large input tolerance is realized regardless of the direction of the signal current.

Embodiment Hereinafter, a current controlled variable gain amplifier according to an embodiment of the present invention will be described with reference to the drawings.

As shown in FIG. 1, 11 is an operational amplifier, 12 is an NPN
transistor, 13 is NPN transistor, 14 is PN
P transistor, 15 is PNP) Lancistor, 16
1 is a controlled current source that flows in, 17 is a controlled current source that flows out,
8 is a fixed resistor, 19 is an AC signal source of magnitude VIN, 20
is the output.

The operation will be explained using FIGS. 1 and 2.

FIG. 2 shows a characteristic diagram of the variable resistance element of the present invention. If the vertical axis is the current l51g flowing through the variable resistance element, and the horizontal axis is the voltage vR across the variable resistance element, then in the saturation region of the element, The voltage versus current of the variable resistance element is approximately proportional, and its bias (ie, resistance value) changes depending on the control current. According to the calculation, the resistance value Rv of the variable resistance element when the control current is 1o is as follows.

Therefore, in FIG. 1, the resistance value of the fixed resistor 18 is R.

The gain Av of this inverting amplifier is as follows.

Therefore, by changing the control current 1o, the gain Av can be changed.

This variable resistance element is shown in FIG.
A control current is applied from the base of PNP transistor 14, and the same amount of control current as that applied is applied to the PNP transistor toaster 14.
This prevents the control current from superimposing the input signal current, which would adversely affect the distortion rate.

In addition, compared to conventional variable resistance elements, this variable resistance element is a combination of a PNP transistor and an NP)1 transistor in parallel, so no matter whether the signal current flows in the positive or negative direction, the variable resistance element will not be saturated. The area expands to approximately the same range of positive and negative values, and the allowable input becomes larger.

As described above, according to this embodiment, by using an inverting amplifier configuration using an operational amplifier and a current-controlled variable resistance element that utilizes the saturation region of an NPN transistor and a PNP transistor, a low distortion rate and a large input tolerance can be achieved. It can be made into a current controlled variable gain amplifier with the following characteristics.

Effects of the Invention As described above, the current controlled variable gain amplifier of the present invention achieves low distortion by separating the signal current and control current, and by combining NPN transistors and PNP transistors. By configuring an inverting amplifier with a variable resistance element and an operational amplifier that have expanded the permissible input, a high-performance current-controlled variable gain amplifier can be created, which is extremely useful in practice.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of a current controlled variable gain amplifier in an embodiment of the invention, Figure 2 is a characteristic diagram of the variable resistance element in Figure 1, and Figure 3 is a conventional current controlled variable gain amplifier. FIG. +VCC・・・Positive power supply, -VER・・・・・・
Negative power supply, 11... operational amplifier, 12.13.
...NPN transistor, 14.15...
・PNP transistor, 16.17... Constant current source, 18... Fixed resistor, 19... AC signal source.

Claims (1)

[Claims] The emitter of the first NPN transistor and the emitter of the second PNP transistor are connected to each other, and the emitter of the first NPN transistor is connected to each other.
The collector of the PN transistor and the collector of the second PNP transistor are connected to each other, and the collector of the first NP transistor is connected to each other.
The base of the N transistor is connected to the base and collector of the third NPN transistor, and the emitter of the third NPN transistor and the emitter of the fourth PNP transistor are connected to the collectors of the first NPN transistor and the second PNP transistor. Furthermore, the base and collector of the fourth PNP transistor are connected to the base of the second PNP transistor, and the connection point between the emitter of the first NPN transistor and the second PNP transistor is connected to the base of the second PNP transistor. connected to the inverting input terminal of the amplifier, and further connected to the collector of the first NPN transistor and the second PN transistor.
A connection point between the collector of the P transistor, the emitter of the third NPN transistor, and the emitter of the fourth PNP transistor is connected to the output of the first operational amplifier, and one end of the first resistor is connected to the output of the first operational amplifier. The other terminal of the first resistor is connected to the inverting input terminal, and the other terminal of the first resistor is used as an input of an AC signal.Furthermore, the non-inverting input terminal of the first operational amplifier is grounded, and the base of the first NPN transistor and the third NPN
The current flowing into the collector and base of the transistor and the second
By making the current flowing out from the base of the PNP transistor and the collector and base of the fourth PNP transistor approximately equal, and changing this current value approximately equally, the distance between the collector and emitter of the first and second transistors can be varied. A current-controlled variable gain amplifier that functions as a resistive element.