JPH0143484B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a differential amplifier composed of insulated gate field effect transistors.

FIG. 1 shows a differential amplifier constructed from conventional insulated gate field effect transistors. P channel transistor (hereinafter referred to as PT) 11 and N channel transistor (hereinafter referred to as NT) 1
2 forms a bias circuit, and 11 is designed to have a higher conductance than 12 under the same bias conditions as a transistor, so its output
VG has a value near the threshold voltage of NT12, and the constant current source transistor 13 of the differential amplification stage and output stage
This becomes a constant voltage bias for 19. NT13,
In the differential amplifier stage consisting of PTs 14 and 15 and PTs 16 and 17, 14 and 15 are a differential input transistor pair, 16 and 17 are a load transistor pair, and 1
4, 16; 15, 17 are both complementary pairs, so a high amplification factor is achieved. 16 has its gate and drain commonly connected so that it is self-biased, and is input to the gate of 17. output transistor
PT18 and constant current source transistor NT19 constitute an output stage, and the output V _D of the differential amplification stage becomes the gate input of 18. Since 18 and 19 are a complementary pair, they have a high amplification factor, and the differential input is effectively amplified by the differential amplification stage and the output stage.

In this conventional differential amplifier circuit, the bias voltage of the constant current source transistors in the differential amplifier stage and the output stage is close to the threshold voltage, so the transistor size is increased to allow current to flow.
The parasitic capacitance of the transistor is also large, and the frequency characteristics cannot be effectively extended by flowing current.

SUMMARY OF THE INVENTION An object of the present invention is to provide a differential amplifier having a structure in which frequency characteristics can be extended.

In order to extend the frequency characteristics by flowing current without increasing parasitic capacitance, it is necessary to input a large constant voltage to the gates of the constant current source transistors in the differential amplifier stage and output stage. In order to stabilize the characteristics of the constant current source transistor, it is required that the effective gate voltage of the constant current source transistor be independent of the power supply voltage.

For this purpose, the differential amplifier of the present invention has a threshold voltage higher than that of the constant current source transistors at the gate of each constant current source transistor connected in series with the differential input transistor pair and the output transistor, and has a common gate and drain. The source-drain voltage of the connected transistor is input.

An embodiment of the invention is shown in FIG. Figure 2 is the first
It is based on a differential amplifier with the configuration shown in the figure, and the bias circuit has been improved. 21, 23 to 29 are shown in Fig. 1
1, 13 to 19, and 22 differs from 12 in that its threshold voltage is higher. To illustrate this, 22 is shown with a wavy line attached to the gate. PT21 is designed to have higher conductance than NT22 under the same bias conditions as a transistor, so the bias circuit output V _G is NT22.
The effective gate voltage of the constant current source transistors 23 and 29 in the differential amplification stage and the output stage is 22 and 23, or 22 and 2.
It is given by the difference in threshold voltage of 9. By ensuring that this threshold voltage difference has no power supply voltage dependence, and by setting this difference voltage to an appropriate size and selecting the transistor size according to the current value, a differential amplifier with excellent characteristics can be achieved. can be achieved.

The transistor 22 with this high threshold voltage is (i) Channel doping (ii) Gate material selection This is done using one of the following methods.

Regarding (i), 22, 23, and 29 are originally formed as transistors with the same substrate concentration, and 2
3, 29 can be channel-doped with donor ions such as ³¹ P ⁺ to lower the threshold voltage, or 22
Increase the threshold voltage by channel doping with acceptor ions, e.g. ^{11B +} .

Regarding (ii), in particular, in the case where polycrystalline silicon is used as the gate material, a gate in which 22 is a P-type impurity and 23 and 29 are N-type impurities is selected.

In either case, the differential input transistor pair 24, 25 can be transistors with the same threshold voltage as 23, 29, or can be transistors with the same threshold voltage as 22.

The differential amplifier of the present invention described above biases the source-drain voltage of the constant current source transistors in the differential amplification stage and the output stage of transistors having the same polarity, high threshold voltage, and whose gates and drains are commonly connected. As a result, the effective gate voltage of the constant current source transistor is increased, and the frequency characteristics are improved by creating a configuration that allows current to flow, and excellent characteristics can be obtained with a simple configuration.

[Brief explanation of drawings]

FIG. 1 shows a conventional differential amplifier. FIG. 2 shows a differential amplifier of the present invention.

Claims (1)

[Claims] 1. A first electrode that connects the source electrode to the first power supply potential.
a constant current source transistor, a differential input transistor pair connected in series to the constant current source transistor, a load element connected between the differential input transistor and a second power supply potential, and the first power supply a second constant current source transistor having a source electrode connected to the potential; and a second constant current source transistor connected between the second constant current source transistor and the second power supply potential and having a gate electrode connected to the differential input transistor and the load element. In a differential amplifier comprising an output transistor connected to a connection point, a source electrode is connected to the first power supply potential, a gate electrode and a drain electrode are commonly connected, and the first and second constant current source transistors are connected to the first power supply potential. a bias transistor having a threshold voltage higher than a threshold voltage; and a bias load element connected between a drain electrode of the bias transistor and the second power supply potential, The constant current source transistor and the bias transistor are transistors of the same polarity,
A differential amplifier characterized in that each gate electrode of the first and second constant current source transistors is connected to the drain electrode of the bias transistor.