JPS5952908A - Variable gain amplifier - Google Patents

Abstract

PURPOSE:To obtain an AGC amplifier of a wide range, which generates no waveform distortion, by inserting one diode or transistor into a general conventional transistor AGC amplifier, by a theory of the variable gain by which a logarithmic transducer and an exponential transducer are cascade-connected. CONSTITUTION:A logarithmic transducer 1 using one diode D1 and an expotential transducer 2 consisting of a transistor TR1 are cascade-connected. An input signal supplied through an input transformer T2 is supplied to the transistor TR1 of the exponential transducer 2 through the logarithmic transducer 1, and an output signal whose gain is adjusted is fetched from intermediate frequency transformer T1 inserted into a collecting circuit of this transistor TR1. A constant-current power source ID determines an operating point of the diode D1, and a constant-current power source Ic/beta determines an operating point of the transistor. A linear amplifier of the variable gain can be constituted by changing each operating point.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Application in Industry In general, AGC (automatic gain control) amplifiers are widely used, such as intermediate frequency amplifiers in radio receivers and automatic recording level adjusters in tape recorders.

The present invention has a simple circuit configuration and does not cause waveform distortion.
The present invention provides a variable gain amplifier for AGC having a wide variable gain range.

<Prior Art> FIG. 1 shows the principle of an AGC amplifier, taking as an example a conventional intermediate frequency amplifier for a radio receiver using a transistor as an amplifying element. Please enter here.

VBE is applied to the base of transistor Trl along with the AGC voltage. Resistor RA + capacitor cA is A G
In the C voltage filter, the average value of the base voltages of transistors 1 to Trl is VBE. rc is the average value of the collector current ++ (is the trust component and becomes the output signal by the intermediate frequency transformer T1. , Vo are determined by physical constants and are approximately 26 nIV.

The following equation is obtained from the flN2+ equation.

FIG. 2 shows the characteristics of the circuit shown in FIG. 1, where the collector current is an exponential function of the base voltage as shown in equation (1). V
Considering the case where I3E is constant, 16 as shown in equation (3)
Since is proportional to IC, the gain can be changed significantly by changing the operating point of the transistor Tr1 depending on the AGC voltage, that is, by changing Ic.

Tsutomu suggested that if VI3E is too large, move the AGC voltage negative to make the IC smaller, and reduce the gain to keep the output signal constant at 1c (problem). In this way, conventional GC amplifiers utilize nonlinearity between the human power output and the output, so as shown in Figure 2, waveform distortion occurs between the human power and the output due to an exponential function, and the human power output becomes larger. It had the disadvantage that the distortion increased significantly.

Means for Solving the Problems> The present invention has been made in view of the above points, and is a method in which the input signal of a transistor amplifier having an exponential characteristic is made proportional to the logarithm of the input signal manually using a logarithmic converter. By converting the signal into a voltage, generation of waveform distortion is prevented, and the variable gain characteristic of the logarithmic converter is also used to obtain a very wide variable gain range.

A description will be given below along with embodiments shown in the drawings. FIG. 3 shows a principle diagram of the variable gain amplifier according to the present invention, where 1 is 1
There is a logarithmic converter using diodes D1, where 2 is 1
- An index converter consisting of a transistor Tr1, in which the logarithmic converter 1 and the index converter 2 are connected in cascade. Here, the input signal supplied via the input quadrance T2 is supplied to the transistor Tr1 of the exponential converter 2 via the logarithmic converter 1, and the gain is adjusted by the intermediate frequency transformer T inserted in the collector circuit of this transistor Trl. The output signal sent is retrieved.

In Fig. 3, 01 and C2 are DC blocking capacitors, II) is a constant current power supply, and this power supply II) is a Tr.
Let the current amplification factor be l. Furthermore, the resistance I<s may be substituted with the output impedance of the human quadrangle T2, or may be eliminated.

Now, if we analyze the operation of the amplifier shown in Fig. 3 in more detail, first the current of the diode D1 is expressed as (1).

Similar to equation (2), it is expressed by an exponential function, and becomes the following equation.

However, KD is a constant determined by the drawing of the diode.

m (Equation 2j and f4) From Equation (5), the voltage of the diode and the base voltage have the same AC component, that is, vBE = Vl) (8
), so it follows from the equation (Li17H8). That is, a variable gain linear amplifier can be constructed by cascading a logarithmic converter and an exponential converter and changing the operating points of each.

Since the human power signal current 15 is the sum of ID and the AC component of the base current (!l), from equation [10], the input signal? current 15 and output signal current l.

Obtain Ic as the relationship. 1 - The exponential characteristic of transistor Trl is canceled out by the logarithmic characteristic of diode I]1, and IC and β5
is in a proportional relationship so that waveform distortion 61 does not occur, and the gain can be varied over a wide range depending on the ratio of 'I) and Io. T.I.
) is very small compared to the IC, equation (II) becomes io-β1S (12), which has the same gain as a normal 1-run amplifier. When I D >1"/p, equation (11) becomes, and the operating waveform in this case is shown in Fig. 4. The input current 1D is converted to the diode voltage VI), and the base of the transistor Tr) However, the output signal current l at that time is proportional to rc as shown in equation (3) or equation (6), so the gain is variable depending on the operating point.ID+IC
No matter what value is used, waveform distortion will not occur. It is known that the characteristics of transistors and diodes expressed by equations (2) and (5) hold true over a range of 109:1 of IC or II), and the principle of Fig. 3 allows an extremely wide variable gain range. It is possible to construct a linear amplifier with .

The negative 'N5 diagram shows an embodiment of the invention in an intermediate frequency amplifier of a ΔM radio receiver. The transistor Tr1゜Quiode ■〕1 constitutes the variable gain amplifier of the present invention.

Resistors Rc and RB constitute an auto-bias circuit for the transistor Tr1. The circuit configuration of the next stage amplifier 3 is omitted. Is RA2 the AGC voltage adjustment resistor I? The detection output is divided into voltages along with AI. When the detection output is small, no current flows through the diode D1, and the transistor PrI becomes β5J.
Operates as a positive amplifier.

When the detection output increases and the AGC voltage becomes negative,
A current ID begins to flow through the ode D1 and passes through the resistor RB.
The base current supplied to CC 7 Nosk Trl decreases, and Ic also decreases. As the detection output increases, ID increases and Ic decreases.

When the quasiode current begins to flow through the resistors RC and RB, making most of the direct current flow through the resistors RC and RB, ID increases, and as the detection output increases, the transistor i''rl
Since the VBE of is further reduced, Ic is further reduced. When the present invention is implemented in the auto-bias circuit of the transistor Trl in this way, the increase in II) and the decrease in IC can be used for variable gain, so that the gain can be changed effectively. Note that the resistance r<B may be zero.

In FIG. 6, the diode D1 in FIG. 5 is replaced by a transistor Tr.
2 is another embodiment of the present invention in which 2 is replaced with 2.

The emitter current of the transistor Tr2 is almost equal to the collector current -β2), and as shown in equation (5), the transistor Tr2 is connected to the logarithmic converter 1 as well as the diode Dl.
It can be operated as Since the AGC voltage can be applied to the base, the ΔGC voltage RA, CA03*
There is an advantage that the pedance can be large.

Note that ("J+I?/" in FIG. 6), a bias voltage is applied to the output of the signal wave to provide delayed AGC.
is omitted and the output impedance of the input quadrature T2 is used instead.

FIG. 7 shows another embodiment applied to an audio amplifier. Its operation is similar to that of the intermediate frequency amplifier described above, and will therefore be omitted.

FIG. 8 shows yet another embodiment in which an operational amplifier AI and a transistor Tr3 constitute a well-known logarithmic conversion circuit. Transistor T1-4 supplies direct current II) to transistor Tr3 according to the AGC voltage,
The operating point of the logarithmic converter 1 is changed. Transistor Tr
The circuit 1 is an ordinary amplifier circuit, and since the output impedance of the amplifier A is very small, it operates as an index converter 2, and its operating point is fixed.

<Effects> The variable gain amplifier of the present invention is based on the principle of variable gain in which a logarithmic converter and an exponential converter are connected in cascade like two, so that a single diode or transistor can be used instead of a conventional general transistor AGC amplifier. By inserting , it is possible to configure a wide range of AGC amplifiers that do not cause waveform distortion.

[Brief explanation of drawings]

Figure 1 is a circuit diagram of a conventional variable gain amplifier for AGC, Figure 2 is a circuit diagram of a conventional AGC variable gain amplifier.
15 is an explanatory diagram of the operation of the same amplifier, FIG. 3 is a principle diagram of the variable gain amplifier of the present invention, and FIG. 4 is an explanatory diagram of the operation of the same amplifier.
The figure is a circuit diagram of one embodiment of the present invention, and FIGS. 6 to 8 are circuit diagrams of other embodiments of the present invention. 1... Logarithmic converter, 2... Exponential converter, Dl-diode, Tr2... Transistor. Agent Patent attorney Aihiko Fukushi (and 2 others)', 3
(・) → Sea 1-de filling in Kamigyo A toshi I No. 6 Figure 54 Jul + Vcc VEE:, ', 8' mind

Claims (1)

[Claims] 1. A logarithmic converter and an exponential converter are connected in series, and the gain between the output of the logarithmic converter and the output of the exponential converter is determined by connecting the logarithmic converter and the exponential converter in series. A variable gain amplifier characterized in that the gain is varied by changing one or both operating points. 21) Current of one diode as logarithmic converter -
2. The variable gain amplifier according to claim 1, wherein the variable gain amplifier is a logarithmic converter whose operating point can be changed by having means for changing the direct current superimposed on the current input by utilizing voltage characteristics. 3. A logarithmic converter whose operating point can be changed by using the emitter current-base emitter bath flexibility of one transistor as the logarithmic converter and having means for changing the direct current superimposed on the small signal current input. A variable gain amplifier according to claim 1.