JPS622711A - Impedance conversion circuit - Google Patents

Abstract

PURPOSE:To perform the conversion of impedance with no distortion by changing an input signal within a dynamic range of the working input of a source follower circuit. CONSTITUTION:An analog input signal VIN is supplied to an input converting circuit 1. Then the output signal VP of the circuit 1 is supplied to a source follower circuit 2. The circuit 2 delivers an output signal VOUT. Here the circuit 1 is equal to an inverse amplifier circuit consisting of an operational amplifier OP1 and resistances R1-R4. While the circuit 2 is provided with N channel MOS transistors MN1 and MN2 connected in series between the power supply voltage VDD and the earth voltage VSS. The working point of the input signal can be shifted or attenuated by deciding a proper circuit constant.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an impedance conversion circuit used when outputting an analog signal.

[Technical background of the invention and its problems]

Generally, two types of circuits are used for impedance conversion: a source follower type and a voltage follower type. A conventional source follower type impedance conversion circuit is shown in Figure 4 (
As shown in a), the power supply voltage ■. 0 and ground voltage■8
It is composed of N-channel MOS transistors MN1 and MN2 connected in series between the two. Analog input signal ■IN is N-channel MO8t-ransistor MN
1, the source of the N-channel MOS transistor MN1 and the N-channel MOS transistor M
An output signal OUT is output from the connection point with the drain of N2. Further, the N-channel MOS transistor MN2, to which a constant bias voltage 8 is applied to its gate, operates as a constant current source.

Now, assuming ■DO)■IN (1), ■O8〉VGS-■th (2)
Therefore, the N-channel MOS transistor MNI always performs saturation operation. However, ■, S is the drain-source voltage, v68 is the gate-source voltage, and ■th is the threshold voltage. Under the above conditions, N channel MO
In order for a constant current to flow through the S transistor MN1, it is necessary that the gate-source voltage ■Gs be constant, and thus the output signal ■. Ul is analog input signal V1
It operates following N.

In addition, the conventional voltage follower type impedance conversion circuit has an operational amplifier OP as shown in Fig. 4(b).
It is composed of 2. In a region where the differential gain of the operational amplifier OP2 is infinite, the differential input terminals of the operational amplifier OP2 are virtually grounded and operate at the same potential. In other words, the analog input signal ■1N and the output signal ■. Matches Uo.

However, in the conventional source follower type impedance conversion circuit described above, as shown in FIG. 5(a), the output signal ■. The operating point of Ul is analog input signal ■1
-The voltage will be lower by 1 (approximately IV) than the operating point, so
When the analog input signal ■1N becomes lower than the voltage vat, the output signal V. U□ can no longer follow the analog input signal VIN and becomes an output signal ■. There was a problem that Uo was distorted.

In particular, the voltage between the analog input signal v1 - normal operation input lower limit voltage Va1, which is the abnormal operation region, and the ground voltage VSS is a constant voltage regardless of the power supply voltage VDD value, and the normal operation input range is the power supply voltage ■. 0 and normal operation input lower limit voltage V81
This is the area between. Therefore, in the low power supply voltage region, the operating input dynamic range becomes significantly narrower and the output signal ■. There was a problem that Uo was greatly distorted.

In addition, in the above conventional voltage follower type impedance conversion circuit, as shown in FIG. 5(b), due to the characteristics of the operational amplifier OP2, the operating input dynamic
Analog input signal that has a narrow range and exceeds that range■1
There was a problem in that the output signal VoU could not follow N and was distorted. Especially analog input signal ■IN
Between the M source voltage ■00 and the normal operation input upper limit voltage ■a2, which is the abnormal operation area, and the normal operation input lower limit voltage ■a3
and the ground voltage SS are constant voltage values regardless of the power supply voltage VDD value, and the normal operation input range is the normal operation input upper limit voltage a2 and the normal operation input lower limit voltage va3.
This is the area between. Therefore, in the low power supply voltage region, the operating input dynamic range becomes significantly narrower and the output voltage V. There was a problem that Uo was greatly distorted.

[Purpose of the invention]

The present invention has been made in consideration of the above circumstances, and an object of the present invention is to provide an impedance conversion circuit that operates without distortion.

[Summary of the invention]

In order to achieve the above object, an impedance conversion circuit according to the present invention has a source follower circuit, and converts the input signal to within the operating input dynamic range of the source follower circuit by attenuating and/or moving the operating point with respect to the input signal. It is characterized by having an input conversion circuit that allows

As a result, the input signal is converted within the operating input dynamic range of the source follower circuit, and impedance conversion is performed without distortion.

[Embodiments of the invention]

A first embodiment of an impedance conversion circuit according to an embodiment of the present invention
As shown in the figure. The analog input signal ■IN is input to the manual conversion circuit 1, the output signal ■ of the input conversion circuit 1 is input to the source follower circuit 2, and the source follower circuit 2 outputs the output signal ■. Output Ul.

An impedance conversion circuit according to a specific example of this embodiment is shown in FIG. 2(a). Input conversion circuit 1 is operational amplifier OP1
and resistors R1, R2, R3, and R4. Analog input signal ■IN is resistor R1
The signal is input to the negative input terminal of the operational amplifier OP1 via the OP1. Power supply voltage■ and ground voltage■8. Resistor R3, R
4 is connected to D, and the connection point between resistor R3 and resistor R4 is operational amplifier O.
Connected to the positive input terminal of P1. The output terminal of the operational amplifier OP1 is connected to the negative input terminal of the operational amplifier OPI via the resistor R2, and the output terminal of the input conversion circuit 1 is connected to the negative input terminal of the operational amplifier OPI.
, is output.

Further, the source follower circuit 2 is connected to the power supply voltage V. 0 and the ground lightning voltage vS, an N-channel MOS transistor MN1. It is composed of MN2. The output signal V of the input conversion circuit 1 is input to the gate of the N-channel MOS transistor MNI, and an output signal (2) is output from the connection point between the source of the N-channel MOS transistor MN1 and the drain of the N-channel MOS transistor MN2. U
□ is output. N-channel MOS transistor MN to which a constant bias voltage v8 is applied to the gate
2 is a constant current source.

Next, the operation will be explained. In this specific example, by appropriately determining circuit constants, it is possible to shift or attenuate the operating point of the input signal. Now, the circuit constants and power supply voltage of this specific example are R1=R2=100KO R3=12.5 to Ω R4=17.5 to Ω (3) VD port=
3v v 5s=ov. Further, the normal operation input lower limit voltage va of the source follower circuit 2 is set to V, =1v (4). Furthermore, the analog input signal VIN has an operating point of 1.5■
, a sinusoidal signal with an amplitude of 1v, VI,4=1
, 5 + sin ωt (5).

Since the differential stage input of the A amplifier OP1 is virtually grounded, the following equation holds true. However, Vb is the positive input terminal voltage of the operational amplifier OPI, and is expressed as follows. Substituting the conditions in equations (6) and (7), we get V
= 3.5 VIN=2 s+nωt[vl
(a) becomes. From this formula (8), V p (In!n) = 1 V, and as the input signal v1 - cropping point moves, V ≦ V
p≦V00 (10).

In this way, the analog input signal ■IN in equation (5) is inverted and amplified by the input signal conversion circuit 1 into the output signal ■, shown in equation (8), of the input signal conversion circuit 1, and this output signal V is supplied to the source follower circuit. The signal is within the operating input dynamic range of 2. Roughly speaking, this output signal V is converted by the source follower circuit 2 into an output signal ``VOUT'' whose operating point follows at a level lower by voltage .

The above analog input signal ■1N, the output signal V of the input signal conversion circuit 1, and the output signal ■ of the source follower circuit 2. Ul
The signal waveforms of each are shown in Fig. 3(a).

Also, in order to attenuate the input signal ■1N, set the circuit constants and power supply voltage to R1 = 100 to Ω, R2 = 50 to Ω, R3 = R4 = 15 to Ω (3)'V RO◎-3v v ss"" o v shall be. Substituting the conditions of equations (3)' and (5) into equations (6) and (7), we get V = 1.5-0.5sin (1) t [vl
(8) '. From this formula (8)′, VP(”ax)=7v, 0.7V p
(1n) -1v, the input signal v 1Hti attenuates and V ≦V, <V
, o (10)'.

In this way, the analog input signal ■1N in equation (5) is inverted and amplified by the input signal conversion circuit 1 into the output signal V of the input signal conversion circuit 1 shown in equation (8)', and this output signal ■,
is a signal within the operational dynamic range of the source follower circuit 2. Furthermore, this output signal V, is the source
The follower circuit 2 causes the operating point to be the voltage V, and the output signal ■ follows at a much lower level. Converted to ol.

The above analog input signal ■1N, the output signal V of the input signal conversion circuit 1, and the output signal ■ of the source follower circuit 2. II
The respective signal waveforms of T are shown in FIG. 3(b).

Note that it is also possible to attenuate the input signal and change the operating point by appropriately determining circuit constants.

Further, an impedance conversion circuit according to another specific example of this embodiment is shown in FIG. 2(b). Input conversion circuit 1 receives control signal φ
, analog switches 81 , 32 , 85 synchronized with
, an analog switch 33 synchronized with the control signal φ,
, 84, 86, capacitors CI, C2, and resistors R5, R6. Analog input signal VI
An analog switch S2 is connected in series between N and the voltage G, and an analog switch S1 and a capacitor C1
The connection point with the input conversion circuit 1 is connected to the output signal VP of the input conversion circuit 1 via an analog switch S3. Capacitor C
1 and analog switch S2 and ground voltage V8
8, an analog switch S4, an analog switch S5, and a capacitor C2 are connected in series.

Also, the power supply voltage V. 0 and ground voltage V88.
, R6 are connected in series, and the connection point between resistor R5 and resistor R6 is analog switch S4 and analog switch S5.
connected to the connection point. Further, the connection point between the analog switch S5 and the capacitor C2 is connected to the output signal V of the input conversion circuit 1 via the analog switch S6.

The source follower circuit 2 is the same as the above specific example, and the output signal V of the input conversion circuit 1 is input to the gate of the N-channel MOS transistor MN1, and this N-channel MOS transistor MN
Output signal V from the source of OS transistor MNI. tl
T is being output.

Next, the operation will be explained. Now, the circuit constants of this specific example are C1=1PF C2=OFF R5=250Ω R6=50Ω (11
) ■mouth 0-3 v ■5s-0■ VG-O■. In addition, the normal operation input lower limit voltage v,1 of the source follower circuit 2 and the analog input signal V1N! If each is set the same as Moe's concrete example, V , = 1v (12) V
IN= 1.5+ sin ωt [vl
(13).

When the control signal φ goes high, the analog switches 81, 82, 85 are turned on and the capacitors C1,
The charges Q1· and C2 respectively charged to C2 are as follows. However, vo is the voltage at the connection point between analog switch s4 and analog switch S5, and is expressed as follows.

Next, when the control signal φ becomes a low level and the control signal φb becomes a high level, the analog switch 83, S4.8
6 is turned on, and one end of capacitor C1 is at voltage VC.
One end of the capacitor C2 is connected to the output signal V of the input conversion circuit 1, and one end of the capacitor C2 is connected to the ground voltage V88.
The other end is connected to the output signal V of the input conversion circuit 1. At this time, capacitors CI and C2 are rendered conductive, causing charge sharing. The output signal V of the input conversion circuit 1 at this time is V, = -(V114-Vo)VC(16)CI +
02. Substituting the conditions of equation (11) into equations (15) and (16), V p = 2 + Sin (t) 'j [V]
(17). From this formula (17), it is always ■ ≦V ≦vDD (19)a
It becomes lP.

In this way, the analog input signal VIN of equation (13) is converted to (17
) is converted into the output signal V of the input conversion circuit 1 as shown in the formula, and this output signal V becomes a signal within the operating input dynamic range of the source output circuit 2. Further, this output signal (2) is an output signal V whose operating point is followed by the source follower circuit 2 at a level lower by the voltage va1. 11
, is converted to

The above analog input signal V11 is the output signal ■ of the manual saw conversion circuit 1, and the output signal ■ of the source follower circuit 2. The respective signal waveforms of Ul are not shown in FIG. 3(C).

As described above, according to this embodiment, it is possible to realize a wide operating input dynamic range of the impedance conversion circuit without expanding the operating input dynamic range of the source follower circuit 2, and to achieve low power consumption. Ii! It can also be operated with voltage.

〔Effect of the invention〕

As described above, according to the present invention, impedance conversion can be performed without causing distortion.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an impedance conversion circuit according to an embodiment of the present invention, FIG. 2 is a circuit diagram showing a specific example of the same impedance conversion circuit, and FIG. 3 is a diagram showing input/output signal waveforms of the same specific example. , FIG. 4 is a circuit diagram showing a conventional impedance conversion circuit, and FIG. 5 is a diagram showing input/output signal waveforms of the same impedance conversion circuit. 1... Input conversion circuit, 2... Source follower circuit, V
lN...input signal, ■,...output signal of the input conversion circuit, vOUT...output signal, vDO...power supply voltage,
v88...Ground voltage, Vv...Normal operation input lower limit voltage, va2al゛a3...Normal operation input upper limit voltage, OPl, OR3...Operation amplifier, CI, C2...Capacitor, R1, R2
. ...R6...Resistor, 81, 82,...
...S6...Analog switch, MNl, MN
2...N channel MOS transistor, φ, φゎ...control signal. Applicant's agent Kiyoshi Inomata Figure 2 (α) Figure 2 (b) "-m-" Figure 3 (α) Figure 3 (b) 0°) Figure 4 3°9

Claims (1)

[Claims] The impedance conversion circuit having a source follower circuit may include an input conversion circuit that attenuates and/or shifts the operating point of the input signal to convert the input signal within the operating input dynamic range of the source follower circuit. Features an impedance conversion circuit.