JPH0328578Y2 - - Google Patents

Description

[Detailed Description of the Invention] A. Field of Industrial Application The present invention relates to a modulation circuit using a MOS transistor as a modulation element.

B. Summary of the invention In a modulation circuit using MOS transistors as modulation elements, the invention consists of a differential transistor pair composed of output MOS transistors in each voltage follower circuit, and modulation between each voltage follower circuit. By providing a MOS transistor for this purpose and allowing a distortion-free drain current to flow through the transistor, it is possible to obtain a modulated output without fluctuations in the DC component and without distortion.

C. Prior Art Conventionally, as a modulation circuit that amplitude-modulates an input signal according to a modulation signal and outputs the resultant signal, a circuit using a MOS transistor as a modulation element as shown in FIG. 5, for example, is generally known. This modulation circuit consists of a transistor 101 and a transistor 10.
2, the source of the source follower transistor 101 is connected to the modulation transistor 102.
connected to the drain of Further, the transistor 101 is used in a saturated region, and the transistor 102 is used in a non-saturated region.

When the input signal V _I is supplied to the gate of the transistor 101, the input signal V _I is modulated by the modulation signal _EM , and the output signal V _O is output from the source of the transistor 101. That is, since the transistor 102 is used in a non-saturation region, the impedance r _d2 seen from the drain of the transistor 102 is equal to the modulation signal E _M
As a result, the input signal V _I is modulated by the modulation signal _EM . The relationship between the input signal V _I and the output signal V _O can be expressed as the following formula.

V _O =r _d2 /r _s1 +r _{d2 V} Iwhere, r _s1 is the impedance seen from the source of the transistor 101.

D Problems to be solved by the invention By the way, in the conventional modulation circuit mentioned above,
The input signal V _I is modulated by the modulation signal _EM , and an output signal V _O is obtained at the source of the transistor 101, but there is a problem that the DC component of the output signal V _O fluctuates. Furthermore, the output impedance of the transistor 101, that is, the r _s1
There is a problem that distortion is likely to occur in the output signal V _O due to the fact that V O has a larger value than that of a bipolar transistor.

Therefore, the present invention was proposed in view of the above-mentioned conventional problems, and the purpose is to provide a modulation circuit that can obtain an output signal (modulated output) without fluctuations in the DC component and without distortion. shall be.

E Means for Solving the Problems In order to achieve the above-mentioned object, the modulation circuit according to the present invention supplies a pair of input signals to both ends of the MOS transistor through each voltage follower circuit, and Each output MOS transistor in each voltage follower circuit constitutes a differential transistor pair, and a modulation signal is supplied to the gate of the MOS transistor between each voltage follower circuit to perform a modulation operation. Features.

F Effect According to the present invention, by supplying a modulation signal to the gate of the MOS transistor provided between each voltage follower circuit, the MOS transistor operates as a variable resistor, and each output in each voltage follower circuit is operated as a variable resistor. Modulated output is obtained from the drain of the MOS transistor.

G. Embodiments Hereinafter, embodiments of the modulation circuit according to the present invention will be described in detail with reference to the drawings. Note that all transistors in the first and second embodiments are MOS transistors.

G-1 Embodiment 1 FIG. 1 is a circuit diagram showing a modulation circuit of a first embodiment. In FIG. 1, the first signal input terminal 1
is connected to the source of the modulation transistor 31 via the first voltage follower circuit 10, and the second signal input terminal 2 is connected to the drain of the transistor 31 via the second voltage follower circuit 20. It is connected. The transistor 31
A modulation signal _EM is supplied to the gate of the . In addition, the voltage follower circuit 1
Each output transistor 16, 26 within 0,20
(indicated by broken lines in the figure) constitute a differential transistor pair, and each transistor 16, 26
The respective drains are connected to a power supply terminal 34 via load resistors 32 and 33, respectively, and are also connected to signal output terminals 35 and 36, respectively.
Note that the transistor 31 needs to be operated in a non-saturation region.

Here, each of the voltage follower circuits 10,
As the circuit 20, for example, a so-called Burton circuit may be used. FIG. 2 shows a specific configuration of the entire circuit when this Burton circuit is used. In addition,
This FIG. 2 corresponds to FIG. 1, and the same reference numbers as in FIG. 1 are used for each part.

First, the first voltage follower circuit 10 (Burton circuit) will be explained. The first signal input terminal 1 is connected to the gate of the transistor 11. These transistors 11 and 12
form a differential transistor pair, and each source is commonly connected to a current source transistor 13. The drains of the transistors 11 and 12 are respectively connected to a power supply terminal 34 via transistors 14 and 15 having a current mirror configuration.
Further, a source floor (output) transistor 16 is connected between the drain and gate of the transistor 12, and the source of the transistor 16 is connected to a current source transistor 17.

The second voltage follower circuit 20 (Burton circuit) is also the same as the first voltage follower circuit 10.
It has a similar configuration. That is, the second signal input terminal 2 is connected to the gate of the transistor 21, and the transistor 21 and the transistor 22 form a differential transistor pair. The sources of the transistors 21 and 22 are commonly connected to a current source transistor 23, and the drains of the transistors 24 and 22 have a current mirror configuration.
25 to the power supply terminals 34, respectively. Further, a source follower (output) transistor 26 is connected between the drain and gate of the transistor 22, and the source of the transistor 26 is connected to a current source transistor 27.

In addition, each of the above-mentioned output transistors 16, 26
A differential transistor pair is constituted by the transistors 16 and 26, and the modulation transistor 31 is connected between the sources of the transistors 16 and 26. Furthermore, the drains of the transistors 16 and 26 are connected to the power supply terminal 3 via load resistors 32 and 33, respectively.
4, and the signal output terminal 35,
36, respectively.

Next, prior to explaining the operation of the modulation circuit of this embodiment, the operation of the MOS transistor as a variable resistor will be explained with reference to FIG. In FIG. 3, the signals (voltages) supplied to the terminals 41 and 43 of the MOS transistor 40 (hereinafter simply referred to as a transistor) are V ₁ and V ₃ , respectively.
Let the signal (voltage) supplied to the terminal 42 (ie, the gate) be _V2 . When the terminal 41 is the source and the terminal 43 is the drain, the gate-source voltage V _GS and the drain-source voltage V _DS are expressed as follows.

V _GS = V ₂ - V ₁ ... V _DS = V ₃ - V ₁ ... In addition, the transistor 40 can be operated in a non-saturation region by satisfying the conditional expression V _GS > V _DS + V _T ... The drain current I _DS at this time is given by the following equation.

I _DS = k {(V _GS − V _T ) V _DS −1/2V _DS ² } ... Here, k is a constant determined by the channel length, channel width, etc., and V _T is the threshold value. It is voltage.

Next, _{V 1 and V 3 are} decomposed into DC component V ₀ and AC _component and V _DS are as follows from formula and formula, respectively: V _GS =V ₂ -V ₀ (1+x) ... V _DS = -2V ₀ x .... Therefore, by substituting these equations into the equation, the drain current I _DS becomes as shown in the following equation.

I _DS = 2xV ₀ (V ₀ −V ₂ +V _T )... On the other hand, if the terminal 41 of the transistor 40 is the drain and the terminal 43 is the source, V _GS and
V _DS is expressed as follows.

V _GS = V ₂ - V ₃ ... V _DS = V ₁ - V ₃ ... In this case, the drain current I _DS corresponding to the above equation is as follows.

I _DS = −2xV ₀ (V ₀ −V ₂ +V _T )...And if V ₀ −V ₂ = V _C (control voltage),
The drain current I _DS is expressed as I _DS = ±2xV ₀ (V _C +V _T ) from the above formula and the above formula. Here, if xV ₀ =△V ₀ , then I _DS =±2△V ₀ (V _C +V _T )... From this equation, it can be seen that the drain current I _DS changes approximately linearly according to the control voltage V _C. This means that the transistor 40 operates as a variable resistance (linear resistance).

The modulation circuit according to the present invention utilizes the fact that the MOS transistor operates as a variable resistor in this way.

Next, the operation will be explained with reference to FIG. First, input the input signal + to the first signal input terminal 1.
V _I is input to the second signal input terminal 2, and the input signal −V _I is
And the modulation signal E _M is applied to the gate of the transistor 31.
are supplied respectively. Then, on the first voltage follower circuit 10 side, the transistor 1
4 and 15 become a so-called active load, and the differential transistor pair 11 and 12 operate, and the output obtained at the drain of transistor 12 is supplied to the source of transistor 31 via transistor 16, and the output of transistor 12 is supplied to the source of transistor 31 via transistor 16. Returned to the gate. On the other hand, a similar operation is performed on the second voltage follower circuit 20 side, and the transistor 2
The output obtained at the drain of transistor 26
is supplied to the drain of the transistor 31 via. At this time, the signal supplied to the source of the transistor 31 becomes approximately equal to the input signal +V _I , and the signal supplied to the drain becomes approximately equal to the input signal -V _I. This is because the output impedances of the transistors 16 and 26 are extremely small due to the feedback effect.

A drain current flows through the transistor 31 according to a signal supplied to the source and drain. This drain current is further varied by the modulation signal _EM supplied to the gate. Therefore, the current flowing through each of the resistors 32 and 33 is a modulation signal.
_EM , and an output signal V ₀ is obtained as a modulated output between signal output terminals 35 and 36.

In this way, since the output signal V ₀ is obtained from the differential transistor pair 16 and 26, the DC component does not fluctuate. In addition, the output impedance of the differential transistor pair 16, 26 is extremely small due to the feedback effect, and a distortion-free drain current can flow through the modulation transistor 31 that operates in the non-saturation region. An output signal V ₀ can be obtained.

G-2 Example 2 Next, a modulation circuit according to a second example will be described with reference to FIG. 4. This modulation circuit is a combination of two modulation circuits of the first embodiment described above, and constitutes a balanced modulation circuit. That is,
Each signal input terminal 51, 52 is connected to a modulation transistor 53 via each voltage follower circuit 60, 70, and is also connected to a modulation transistor 54 via each voltage follower circuit 80, 90. has been done. Each output transistor 61, 71 in each of the voltage follower circuits 60, 70 and each output transistor 8 in each voltage follower circuit 80, 90
Reference numerals 1 and 91 each serve as a differential transistor pair. The above transistor 71 and transistor 81
Both drains of the transistor 61 are connected to a power supply terminal 56 via a load resistor 55.
and the drains of transistor 91 are both connected to power supply terminal 56 via load resistor 57. Further, each of the load resistors 55 and 57 is connected to signal output terminals 58 and 59, respectively.

The input signals +V _I and -V _I are supplied to the signal input terminals 51 and 52, respectively, and the modulation signals +E _M and -E _M are supplied to the gates of the transistors 53 and 54, respectively, resulting in modulation. The operation is performed so that an output signal V ₀ is obtained between the signal output terminals 58 and 59. Similarly to the modulation circuit of the first embodiment described above, the modulation circuit of this embodiment can also obtain an output signal V ₀ without fluctuation in the DC component and without distortion.

H. Effects of the invention As is clear from the description of the embodiments described above, according to the invention, each output MOS transistor in each voltage follower circuit constitutes a differential transistor pair, and an output signal is extracted from this transistor pair. Therefore, it is possible to obtain a modulated output without fluctuations in the DC component. In addition, the output impedance of the output MOS transistor mentioned above is extremely small due to the feedback effect, and a distortion-free drain current can be passed through the modulation MOS transistor that operates in the non-saturation region, so a distortion-free modulation output can be obtained. be able to.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a first embodiment of the modulation circuit according to the present invention, FIG. 2 is a circuit diagram showing a specific configuration when a Burton circuit is used as the voltage follower circuit in FIG. 1, and FIG. FIG. 3 is a circuit diagram for explaining the operation of a MOS transistor as a variable resistor, and FIG. 4 is a circuit diagram showing a second embodiment of the modulation circuit according to the present invention. FIG. 5 is a circuit diagram showing a conventional example of a modulation circuit. 10, 20, 60, 70, 80, 90... Voltage follower circuit, 16, 26, 31, 53,
54, 61, 71, 81, 91...transistor.

Claims (1)

[Claims for Utility Model Registration] A pair of input signals are supplied to both ends of the MOS transistor through each voltage follower circuit, and each output signal in each of the voltage follower circuits is
A modulation circuit comprising a differential transistor pair composed of MOS transistors and configured to perform a modulation operation by supplying a modulation signal to the gates of the MOS transistors between the respective voltage follower circuits.