JPS5831765B2 - Differential amplifier circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a differential amplifier circuit using insulated gate transistors (MOSFETs).

With the application of MOSFETs to various circuits, there is a demand for a differential amplifier circuit having a simple circuit configuration.

The differential amplification circuit uses the cathode side electrodes of a pair of transistors, that is, M
The source electrode of an OSFET and the emitter electrode of a bipolar transistor are connected in common, and these electrodes are driven by a current source.

A conventional constant current source supplies a constant base current from a power supply to a bipolar transistor, and uses pressure characteristics to determine the percentage current between the transistor's collector and emitter electrodes.

This constant current circuit not only has a complicated circuit configuration for supplying a base current, but is also unsuitable for combination with a MOSFET circuit.

An object of the present invention is to provide a differential amplifier equipped with a constant current circuit having a simple configuration and excellent constant current characteristics.

According to the present invention, the drain electrodes of the first and second insulated gate transistors are connected to one terminal of a power supply through their respective loads, input is applied to each gate electrode, and the source electrode is commonly connected to the first and second insulated gate transistors. The source electrode of the third transistor is connected to the drain electrode of a third insulated gate transistor having a channel length longer than that of the transistor, the third transistor is driven to the saturation region, and the source electrode of the third transistor is connected to the other terminal of the power supply. A differential amplifier circuit is obtained, which is characterized in that it is connected to.

FIG. 1 is a circuit diagram showing a differential amplifier according to an embodiment of the present invention.

In this embodiment, the source electrodes of load MOSFETs Q3 and Q4 are connected to the drain electrode of the first MOSFET Q+ and the drain electrode of the second MOSFET Q2, respectively.
+ drain electrode and gate electrode to power supply E.

Connect to one terminal VDD of.

The gate electrodes of MOS FET Ql and Q2 are connected to input terminals REF and IN, respectively, and a predetermined input voltage is applied thereto.

In addition, the source electrodes are commonly connected to the third MOSFET Q5.
Connect to the drain electrode of

The gate electrode of MOSFET Q5 is connected to the power supply terminal VDD, and the source electrode is connected to the ground terminal GND.

Further, the basic gate electrodes of all transistors Q1 to Q2 are commonly connected to a ground terminal.

The drain electrodes of MOSFETs Ql and Q2 are led out to the output terminals OUT and OUT, respectively, and are connected to the input terminal I
Obtain the complementary output signal of the signal applied to N.

Here, MOS FET Q5 has a longer channel length than other MOSFETs, as will be described later.

In the present invention, the base gate electrodes of MOSFETs QIQ2 and Q5 are commonly fixed to the ground potential as shown in FIG.

For this reason, in MOSFET Qt, Q2, due to the rise in the source potential of these MOSFETs,
In each MOS FET Ql, Q2, a relative back gate bias is generated between the base gate electrode and the source.

Due to this back gate bias, MOS FET Q7. The threshold voltage of Q2 causes the above-described variation ΔVTHΔ■TH2 to exist.

Such ΔVTH and ΔvTH are N-channel MO8FET
has a positive value with respect to increase in source voltage.

Therefore, M tries to transition from a non-conducting state to a conducting state.
The threshold voltage of OS FET Ql or Q2 increases as its source potential increases.

This increase in threshold voltage contributes to high-speed, high-sensitivity operation, as detailed below.

※※ In other words, the differential amplifier consists of a pair of differential MO8FETs Ql,
When one side of Q2 is completely cut off and the other side is in a deep conduction state, the largest amount of level change and charge transfer is required, resulting in slow speed.

For this reason, it is necessary to effectively operate the MOS FET Ql, Q2 with a small amplitude to increase the speed of operation.

In the present invention, the threshold voltages of the MOSFETs Ql and Q2 rise as described above due to the back gate bias effect, so the thresholds approach the input signal level, and one of the differential MO8FETs enters a deep conduction state. At the same time, the other MOSFET is prevented from entering the complete cut-off region in a complementary manner.

Therefore, the output current levels of the differential MO8FETs are always controlled in a complementary manner, and they are operated in a small amplitude state.
High-speed operation is achieved.

If such complementary and small amplitude current control is not performed, one MOSFET will be cut off, and the other MOSFET Q5 will suppress the current value, resulting in saturation and direct distortion. Not desirable.

Furthermore, an important point when discussing high-speed operation is carrier mobility μ.

Generally, the transfer conductance gm of a MOSFET is expressed as follows, and if μ is constant, it changes depending on the gate voltage condition Vg, but in reality μ is a function of the induced charge.

That is, considering in the - dimension, if the depth direction is X, the possible charge density in the inversion layer is Qn, and the channel conductance between the gate and source is g, the effective mobility μ is Qn'=Co (VG+VT) (Co is oxide film capacitance), tt=I L/W-g/Co (VG-VT
) becomes I.

As is clear from this equation, the mobility μ is the gate voltage V.

The larger the value, the smaller the value. Therefore, in order to increase mobility and perform high-speed operation, it is important to reduce (VGVT).

For this reason, in the present invention, an effective increase in vT reduces (■G-■T), which contributes to increasing carrier mobility.

Thus each MO8FET Ql according to the invention. Q2
, Q5, a differential amplifier circuit capable of extremely high-speed operation can be realized.

In the circuit of this embodiment, MOS FET Q, ~Q5 all operate in the same operating mode, as in a practical circuit of this type, and MOS FET Ql
.

When the gate voltage of Q2 is at most the power supply voltage, M
Find the conditions to obtain constant current characteristics of OSFET Q5.

To obtain this characteristic it is necessary to operate in the pentode region.

The drain voltage of MOSFET Q5 is V8, the gate threshold voltage is VTH, the gate threshold voltage of IVIO8FET Ql or Q2 is vTH, the change due to increase in VTH2 and source voltage is ΔvTH, ΔVTHMOSFET
VTH7VTHj”VTR that of Q2 and Q4
When 3 4 j''VTH, the constant current operating conditions are expressed as and from these conditions.

Since all the voltage values in equations (4) and 5) have the same polarity, there is a relationship between the steady threshold voltages.

Also, load MOSFET Q3. When exciting the gate electrode of Q4 with a voltage that increases the threshold voltage from the power supply voltage, or when replacing these with other loads such as resistive elements, VTH2ΔvTH, vTH, and ΔvTH are not included in the conditional expression 3 3 4
4, so it becomes .

Therefore, in this embodiment, it is a necessary condition that the gate threshold voltage of MO8FETQ5 is higher than that of MO8F''ETQl, Q2.

This condition applies to MOSFET Q, ~Q4 and MOSFET
This is achieved by using gate insulating films with different structures for Q5.

In a preferred example, all MOSFETs are N-channel type formed on a silicon substrate with a specific resistance of 3Ω, and the MOSFETs are
ET Q.

As the gate insulating film for MOSFET Q4, 300 to 400 silicon dioxide is thermally oxidized and then an alumina film of 1500 to 2000 is grown in vapor phase. For MOSFET Q5, an alumina film is simply grown in vapor phase.

The gate threshold voltages at this time are +1.0 to +1.
5 volts and +5 to +7 volts.

FIG. 2 shows the effect of the embodiment shown in FIG.

In order to improve constant current characteristics, MOSFET Q50
Channel length is 20μ, other transistors are 5μ
Then, the pentode region of drain current − drain voltage ■
The slope of the constant current saturation characteristic is MOSFET Q5
The characteristic 21 of is smaller than the characteristic 22 of other transistors.

Since this slope is an effect of channel length modulation due to an increase in drain voltage, it is reduced by making the channel length larger than the depletion layer extending from the drain region, thereby obtaining better constant current operation.

For this reason, it is preferable to make the MOSFET Q50 channel length longer.

The one-sided transistors Q1 to Q4 are designed to have small channel lengths of about 5 to 12.5 microns to reduce their dimensions.

In this embodiment, a DC bias is applied to the gate electrode of the MOS FET Ql to conduct current S1,
While the potential of the input signal that provides the input signal to the gate electrode of MOSFET Q2 is lower than the gate threshold, the current ■s1 entirely flows to MOSFET Ql.

When the potential of the input signal increases, MOSFET Q5 becomes conductive and the constant current ■s begins to flow in a shunted manner.Finally, when the drain voltage VS of MOSFET Q5 increases and the change in gate threshold voltage ΔVTH1 increases, M O
S F E T Q+ becomes blocked.

These current changes provide an output signal as a potential change at the output terminal OUT.

Since the current changes in MOSFETs Ql and Q2 are complementary, complementary output signals are obtained at the output terminal OUT from the drain electrode of MO8FET Q2.

According to the embodiment of the present invention described above, a differential amplifier with excellent characteristics can be realized with a simple circuit configuration, and is effective in improving the versatility of MO8 type integrated circuits.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of an embodiment of the present invention, and FIG. 2 is a static characteristic diagram for explaining the operation of the embodiment of FIG. Qz, Q2, Qa, Q4, Q50UT, O
UT is the output terminal, GND is the ground terminal. is MOSFET, IN is input terminal,

Claims (1)

[Claims] 1 Connect the drain electrodes of the first and second insulated gate transistors to one terminal of a power supply through their respective loads, apply an input to each gate electrode, and share the source electrode with the first and second transistors. connecting the source electrode of the third transistor to the drain electrode of a third insulated gate transistor having a large channel length, driving the third transistor into a saturation region, and connecting the source electrode of the third transistor to the other terminal of the power supply; A differential amplifier circuit characterized in that the potentials of the base bodies of the first to third transistors are set to the same constant potential.