JPS6052603B2 - differential amplifier circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a differential amplifier circuit used in an AM receiver,
More specifically, the present invention relates to a differential amplifier circuit that operates without problems such as increased distortion or decreased output when a strong AM signal is input.

The differential amplifier circuit used in AM receivers is usually
It is configured as shown in Figure 1, and the voltage applied to terminals 3 and 4 attached to the bases of transistors 1 and 2 is
, and V, the difference voltage ΔV (■V, -V.

) is amplified, and output signals are taken out from output terminals 5 and 6 provided at the collectors of the respective transistors. In addition,
7 and 8 are load resistances of transistors 1 and 2, 9 and 10 are emitter resistances, 11 is a power supply terminal, and 12 is a direct current source. 2a to 2c are diagrams showing the input/output characteristics of such a differential amplifier circuit, in which the horizontal axis in FIG. 2a is the differential voltage ΔV, and the vertical axis is the collector current Icl of the transistor 1. As is clear from this figure, in a normal differential amplifier circuit, regardless of the value of the DC current flowing into the emitter circuit from the DC current source 12, if the absolute value lΔV1 of the differential voltage exceeds a predetermined value, the input/output will be linear. The power is rapidly lost, and the output saturates. Therefore, if lΔVl exceeds a predetermined value and an input signal is applied between terminals 3 and 4,
Distortion increases rapidly. For example, A as shown in Figure 2.
When the M input signal 5, is applied, the output signal 50 is the second
As shown in Figure c, not only is the distortion increased, but part of the modulated signal is also lost. This caused a fatal problem in that the distortion increased and the output decreased in the receiving device. The present invention has been made in view of such problems in differential amplifier circuits, and it connects the emitter common connection point of the differential amplifier circuit and the power supply terminal via a resistor, and also connects the emitter common connection point of the differential amplifier circuit and the power supply terminal through a resistor. This is achieved through a current source whose current changes monotonically in the opposite relationship as the average value of the output voltage output to each output terminal or the average value of the difference between each output voltage increases or decreases. The feature of the differential amplifier circuit of the present invention lies in that the input/output characteristics are changed by connecting the output terminal to the current output terminal to prevent saturation of the output.

The present invention will be explained below with reference to the drawings.

FIG. 3 is a diagram showing the configuration of the differential amplifier circuit of the present invention,
As shown in the figure, a common emitter connection point X of transistors 1 and 2 is connected to a power supply terminal 11 via a resistor 13, and is also connected to a current source terminal 15 via a current source 14 whose current changes. This is different from the conventional differential amplifier circuit shown in FIG.

In the differential amplifier circuit of the present invention having such a configuration, the collector currents of transistors 1 and 2 are set to I. l and I
. 2. The current of the current source 14 is I.

, the value of resistor 13 is R13, and the current flowing through this resistor is 11.
3. The voltages applied to terminals 3 and 4 are Vl, V2, respectively.
The difference voltage (V1-V2) between the two is Δ, the saturation current of the transistor is L1, and the power supply voltage is CO. ．． Assuming that the voltage at point X is Vx and that the values of resistors 9 and 10 are 0 ohm, the currents 1011C2 and 1013 are calculated as follows. From the first equation, if we substitute the fifth equation into the second equation, if we substitute the fifth equation into the third equation, and if we further substitute the sixth and seventh equations into the fourth equation, the base emitter of transistor 1 The voltage between V8.

If BO1 is 1, then BO1 is expressed as follows, and the 8th equation can be rewritten as follows.

Here, k is Boltzmann's constant, T is absolute temperature, and q is electron charge.

As is clear from this equation, the voltage V1 applied to terminal 3
is large, and as a result ΔV becomes large, even if Exp(
-qΔV/KT) ~ 0, 101 can be increased due to the effect of 1/R13 in the second term on the right side, and its saturation can be prevented by setting the value of resistor 13 to an appropriate value. It becomes possible.

Note that when the values of emitter resistors 9 and 10 are not O ohm, the effect of expanding the input dynamic range is achieved, but the 9th equation becomes complicated; Able to take on roles.
By the way, when the resistor 13 does not exist, R of the ninth equation
13 can be considered to be infinite, and only the first term on the right side of Equation 9 remains, and Equation 9 becomes an equation that calculates the collector current of a normal differential amplifier circuit.

Therefore, if a large signal (ΔV>KT/q) such that Exp(-QAV/KT)~0 is input, then I. , and no further increase is possible. On the other hand, in the differential amplifier circuit of the present invention, that is, the circuit in which R13 is present, when a large signal of Exp(-qΔ/KT) to 0 is input, the ninth equation becomes.

Therefore, even if Δ becomes large, that is, V1 becomes large, ICl can increase according to 1/R13 of the second term on the right side. Qualitatively, it can be explained as follows.

For input signal (V, 〉V2), voltage at point X. is V
1, and as V1 increases, V
x also increases and transistor 2 is cut off. At this time, the algebraic sum of 101 and 113 is the current 1 of the constant current source 14.
become. When 1 further increases, the current 113 [113=(VcC-x)/R13] decreases by the amount that Vx increases. The sum of 113 and 113 is I. As a result, ICl increases. In this way, the resistance R
The presence of 13 can prevent the saturation of .
Note that it is important to set the value of R13 to a value that establishes a relationship that is smaller than 113. However, it is difficult to completely prevent the saturation of the resistor with only this resistor, and therefore, in the present invention, the constant current is varied in accordance with the output voltage. This will be explained with reference to FIG.

FIGS. 4a to 4c are diagrams showing the input/output characteristics of the differential amplifier circuit of the present invention. Figure 4a shows the result obtained by calculating Equation 9 using L as a parameter.

The voltage of the current source 14 of the differential amplifier circuit of the present invention shown in FIG. When the input voltage is changed in the direction of decreasing, the collector current of transistor 1 with respect to the input voltage ΔV. The characteristic of 1 changes from A to B, and the input voltage Δ at which 101 becomes zero
V changes in the positive direction.

Therefore, when a relatively large AM signal Si as shown in FIG. 4b is applied as the input signal ΔV, the IO
The input/output characteristics are changed from A to B by decreasing I. If the saturation of ICl is prevented, the output current characteristics of ICl will be such that an output signal as shown in FIG. 4c is obtained. Note that when the input/output characteristics are changed in this way, a portion of the carrier wave is cut off, as is clear from the diagram. However, since the fundamental frequency component remains, there is no problem when performing frequency conversion. Furthermore, in order to reduce the delay when a large signal is applied as an input signal, for example, a known AGC circuit that utilizes the signal output from the output terminal 5 or 6 of the circuit shown in FIG. 3 may be used. All you have to do is control.

FIG. 5 is a diagram showing a specific configuration of an AM receiver converter configured by applying the differential amplifier circuit of the present invention described above.
0 and 21 are transistors that constitute a differential amplifier, respectively, and a terminal 22 is connected to the bases of transistors 16 and 19.
An AM signal is applied to the base of the transistor 21, an oscillation signal is applied from the terminal 23 to the base of the transistor 21, a DC voltage is applied from the power supply terminal 24 to the transistors 17 and 18, and a DC voltage is applied from the terminal 25 to the base of the transistor 20. be done.

26 and 27 are resistors that connect the emitter common connection point of the differential amplifier to the power supply terminal 24, 28 is a current source, 29,
30 is a transistor forming a current mirror; 31 is a transistor that sinks the current of the current source 28 to change the collector current of the transistor 29; 32 is a positive transformer;
33 is an IF filter, 34 is an intermediate frequency amplification circuit, 35 is an audio signal output terminal, 36 is an N°C signal output terminal, and 3
7 and 38 are resistors and capacitors for smoothing N°C signals.

In the circuit having the above configuration, the AM signal is converted to an intermediate frequency signal of 455 KHz by applying the current signal to the terminal 22 and the oscillation signal to the terminal 23, and the IF transformer 32 and the IF It passes through the filter 33 and the intermediate frequency amplification circuit 34 and outputs it to the terminal 3 as an audio signal.
Hail gate on 5.

Further, the AGC signal applied to the terminal 36 is smoothed by a smoothing circuit composed of a resistor 37 and a capacitor 38 and applied to the base of the transistor 31. The current supplied from the current source 28 is inputted into the current source 28. Therefore, when the AGC signal becomes large, transistor 3
1 increases, the collector current of transistor 29 decreases, and the collector current of transistor 30 forming a current mirror with this transistor also decreases. Therefore, the transistor 20,2
1's collector current also decreases, L of the differential amplifier circuit formed by transistors 16 and 17 and transistors 18 and 19 decreases, and its input/output characteristics change from the characteristic shown by A in FIG. 4a to B. It changes to the characteristics shown in
Output saturation is prevented. As is clear from the above explanation, the differential amplifier circuit of the present invention can prevent saturation of the modulated output even when a strong AM signal arrives, and can suppress an increase in distortion and a decrease in output. , it becomes possible to realize a high-performance AM receiver that operates reliably even in strong electric fields.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a conventional differential amplifier circuit, FIGS. 2 a to c are diagrams showing input/output characteristics of the differential amplifier circuit shown in FIG. 1, and FIG. 3 is a diagram showing a differential amplifier circuit of the present invention. Figure 4a shows the configuration of
-c is a diagram showing the input/output characteristics of the differential amplifier circuit of the present invention, and FIG. 5 is a diagram showing the specific configuration of an AM receiver converter constructed by applying the differential amplifier circuit of the present invention. . 1, 2, 16, 17, 18, 19, 20, 21...
Transistor for differential amplifier, 3, 4, 22... Input terminal of differential amplifier, 5, 6... Output terminal of differential amplifier, 7, 8... Collector load resistance, 9,
10... Emitter resistance, 11, 24... Power supply terminal, 12, 14, 28... Current source, 13,
26, 27... Resistor for preventing saturation of differential amplifier, 15
... Current input terminal, 23... Outgoing signal application terminal, 25... DC voltage application terminal, 29, 30...
...Transistor for current mirror configuration, 31.
...Transistor for current branching, 32...
・IF transformer, 33...IF filter, 34...
...Intermediate frequency amplification circuit, 35...Audio signal output terminal, 36...M°C signal output terminal, 37...
...Resistor for AGC signal smoothing, 38...Capacitor for M°C signal smoothing.

Claims (1)

[Claims] 1 The emitters of a first transistor to which an AM signal is applied to the base and a second transistor to which an AM signal or a DC voltage of opposite phase to the AM signal is applied to the base are commonly connected, and at least one of the transistors is connected in common. An output terminal is attached to the collector of the transistor, and the emitter common connection point is connected to a current source whose current monotonically decreases as the AM signal input increases, and is connected to a power supply terminal via a resistor. A differential amplifier circuit characterized in that the input/output characteristics change to input/output characteristics that prevent output saturation due to a decrease in the current of the current source.