JPS61274421A - Analog switch - Google Patents

Abstract

PURPOSE:To obtain the titled switch of a low offset voltage and also a low on-impedance by bringing transistors to an transverse-parallel connection to each other, and making an operating bias current flow to bases separately. CONSTITUTION:Transistor TRs 101 and 102 for constituting an analog switch 100 are brought to the transverse-parallel connection, and to each base, a bias circuit 50 and a switch controlling circuit 150 are connected. When turning on the switch 100, TRs 151, 152 are turned off by a control input terminal 155. As a result, the TRs 101, 102 conduct by a bias current by constant-current sources 51, 52, and a signal source 10 and a load terminal 25 become a conducting state. In this state, the TRs 101 and 102 are brought to the transverse-parallel connection, and a bias current is supplied independently to each base electrode, therefore, on TR is operated as a forward direction TR irrespective of the polarity of the difference voltage of a signal source and a load, and shows a small equivalent on-resistance. In this way, two base bias currents flow to the signal source as a reverse direction operation, and an offset voltage becomes small value.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a transistor switch π, and more particularly to a low offset voltage, low impedance analog switch.

[Background of the invention]

Conventionally, a so-called reverse-operation switch that flows a base current to the collector side of a transistor has been known as a low-offset voltage analog switch (Processing Edition: Industrial Measurement Technology System 12 Signal Conversion and Data Processing (Nikkan Kogyo, 1973). −
9) P2S5 transistor chopper). However, the current amplification factor of the transistor during reverse operation is extremely low, so for example! When using the bidirectional switch as a capacitive load sample hold switch as shown in Figure 3, vc is:
The signal current between collector and emitter when turned on becomes hFl in the opposite direction of the transistor and cannot be increased, resulting in 1 in Figure 2.
As shown in the figure, the charging time was long and the speed could not be increased.

[Purpose of the invention]

An object of the present invention is to provide an analog switch with low offset voltage and low on-impedance.

[Summary of the invention]

The present invention is based on the fact that the reverse operation of a transistor switch has a high on-impedance but a low offset voltage, and the forward operation has a high offset voltage but a low on-impedance (large current). By connecting them in parallel and allowing operating bias currents to flow through the bases individually, it is possible to obtain the combined characteristics of low on-impedance and low offset voltage.

[Embodiments of the invention]

An embodiment of the present invention will be explained below with reference to FIG. 1st
The figure shows an example of the configuration of a sample and hold circuit, in which a transistor analog switch 100 is connected between a signal source 10 and a load capacitor 20. The switch 100 is composed of transistors 101 and 102, and the collector and emitter are connected in antiparallel to each other. The base VC of each transistor 101 and 102 is a pass circuit (7 pass circuits).
ti source) 50 and a switch control circuit 150 are connected.

That is, the base VC of the transistor 101 is the constant current source 51 and the collector power of the transistor 151, and the base VC of the transistor 102 is the constant current source 52 and the transistor 1.
52 collectors are connected.

In Figure 1, to turn on the analog switch,
Transistors 151 and 152 are connected to the control input terminal 155.
Turn off. Then, the bias current from the constant current sources 51 and 52 is applied to the transistor 10 of the analog switch 100.
1,102 flows through the base of transistor 101
, 102 are made conductive, the signal source 10 and the load terminal 20 are brought into conduction, and the load capacitor 20 is charged with the voltage of the signal source io.

Here, the transistors 101 and 102 are connected in anti-parallel, and each base electrode VC independently supplies a bias current, so regardless of the polarity of the differential voltage between the signal source and the load, one transistor is always connected. operates as a forward transistor with a normal base current flowing to the emitter side, a large collector current flows, exhibiting a small equivalent on-resistance, and the load capacitor 20 is rapidly charged to the terminal voltage of the signal source 10. Finally, the two base bias currents flow in the opposite direction through the collector side to the signal source, reducing the offset between the collector and emitter of the transistor.
It exhibits an extremely small value of 1 my or less.

FIG. 2 shows charging characteristics corresponding to the sample and hold circuits shown in FIGS. 1 and 3. In the circuit shown in Figure 3, the signal current is hyz times the base bias current of the reverse transistor, so it cannot be increased, resulting in a situation like 1 in Figure 2, but in the circuit shown in Figure 1, there is always a pace current during the charging process. Since it operates as a forward direction transistor that flows to the emitter side, it exhibits a high
As shown in figure 2, it charges quickly. In other words, the difference in response between 2 and 1 in Figure 2 is the order of the switch transistors.

This corresponds to the opposite difference in amplification factor.

In one embodiment of the present invention, the transistor switch is an NPN transistor, and the bias current source is a constant current source, but they may also be PNP transistors, bias resistors, or the like.

〔Effect of the invention〕

According to the present invention, since the switch transistor can perform a combined forward and reverse operation, it is possible to obtain an analog switch that has both low on-resistance and low offset voltage, and has the effect of achieving high speed and high precision.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of an embodiment using an analog switch of the present invention, FIG. 2 is a characteristic diagram showing electrical characteristics, and FIG. 3 is a circuit diagram showing an example of a conventional analog switch. 10... Signal source, 20... Load capacitor, 50...
...Bias current source, 100...analog switch,
150th 1st m V10th 2nd m House 30 Vsu

Claims (1)

[Claims] 1. Connect a pair of transistors with collector electrodes and emitter electrodes connected in anti-parallel to each other between the signal source and the load,
An analog switch characterized in that a current bias source and a shunt switch are provided on each base electrode of the transistor, and the transistor is switched on and off by controlling the shunt switch.