JPH0326006A - Constant current circuit - Google Patents

Abstract

PURPOSE:To output a constant current with a current ratio of 1:N without causing a large parasitic capacitance by forming an output circuit with a current mirror circuit. CONSTITUTION:The emitter area, that is, the size of transistors(TRs) Q2, Q3 is selected as a multiple of N in comparison with the size of TRs Q1, Q4-Q6, the TRs Q2, A5 form a 1:N current mirror circuit 6 and the TRs Q4, Q6 form the 11 current mirror circuit 7. Then a constant current is outputted from the TRs Q5 and Q6 and the ratio of the collector currents of the TRs Q5 and Q6 is 1:N. Thus, while the size of the Tr Q6 is the same as the size of the TR Q5, a constant current of a multiple N is supplied and the parasitic capacitance between the collector of the TR Q6 and ground is not increased.

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to a constant current circuit having a load current ratio of 1:N suitable for semiconductor integrated circuits.

Constant current circuits that supply a conventional technical reference current and a current N times the current are often used as constant current supply means for differential amplifiers and the like in semiconductor integrated circuits.

FIG. 3 shows a conventional example of a constant current circuit having such a current ratio of 1:N.

In this circuit, the size of transistors (T,) and (T2) is set to 1:N, a common bias voltage v1 is supplied to their gates, and the ratio of the values of resistors (Rl1) and (R+z) is set to N:l. By setting transistors (T+) and (T2
) is taken as INN.

FIG. 4 shows a specific application example of such a constant current circuit (2o). First, the constant current circuit (2o) is connected as shown in the figure, and its output current is set to the first .
It is supplied to the second differential amplifier (21) (22). 1st
The differential amplifier (21) includes a pair of differential pair transistors (T3
) (T4) and a load resistance (Raff). An alternating current signal applied from the input terminal (23) to the base of one differential pair transistor (T,) is derived from the collector of the other differential pair transistor (T4) to the output terminal (24). On the other hand, the second differential amplifier (22) has a similar structure, and the AC signal input from the input terminal (25) is applied to the base of one differential pair transistor (2).
It is taken out from the collector of the other differential pair transistor (T.) to the output terminal (26). (R.) is the load resistance of the second differential amplifier (22). Also, (27) (28)
are the other differential pair transistors (T.) (Th)
It is a constant voltage source that provides a fixed bias to the base of the

Sumi 1 tatami mat plastic butt resolution 32 ri 1! However, in the conventional constant current circuit described above, if we focus on the transistor (T2) that sucks a current N times the reference current, the size of the transistor (specifically, the area of the design)
Since it is multiplied by N, the parasitic capacitance between the collector and ground is larger than that of the transistor (T,) that carries the reference current. Therefore, in Fig. 4, the first differential amplifier (21) which has a constant current N times the reference current has a problem that the frequency characteristics are worse than the second differential amplifier (22) which uses the reference current as the load current. there were.

The present invention has been made in view of these points, and an object of the present invention is to provide a novel constant current circuit devised to output a constant current of 1:N without generating a large parasitic capacitance. ．． Means for Solving the Problems A constant current circuit of the present invention which achieves the above object includes a first transistor with an emitter area of S and a size NS whose emitter area is N times that of the first transistor. a first circuit formed by cascade-connecting second transistors;
a second circuit comprising a third transistor having a size S and a fourth transistor having a size S connected in cascade; means for applying the same drive voltage to the first and second circuits; a fifth transistor of the size S forming an output side transistor of a current mirror circuit using the transistor; and a sixth transistor of the size S forming an output side transistor of the current mirror circuit using the fourth transistor as an input side transistor. and is configured to output a constant current from the fifth and sixth transistors.

In this kind of precept, first of all. The current flowing in the second circuit will be the same. If this current is Nl, then the second
Nl flows through the transistor, but since the size of the fifth transistor is 1/N of that of the second transistor, the current flowing is l (reference current). On the other hand, Nl also flows through the fourth transistor, but the sixth transistor, which forms a current ξ error circuit with this, has a size S
Therefore, Nl flows. Thus, the sixth transistor outputs a current N times the reference current I, but since the size of the sixth transistor is S, which is not large, the parasitic capacitance generated at its collector does not increase. The following describes the embodiments of the present invention shown in the drawings. do.

In Figure 1, (1) represents a first transistor (Q+) with an emitter area (hereinafter referred to as "size") of S, a second transistor (0■) with a size N times S, and a resistor ( R
1) (RZ) connected as shown in the figure, and is connected between the power line (3) and the ground line (4). (2) is the third transistor (Q
, ), a fourth transistor (Ro4) of size S, and a resistor (Ih) (R,) are connected as shown in the figure, and the second circuit is connected as shown in the figure, and is similar to the first circuit (1). It is connected between the power supply line (3) and the ground line (4).The bases of the first transistor (Q,) and the third transistor (Q3) are commonly connected to a constant voltage source (5), A bias voltage (vg) is applied.Next, (Q5) is the fifth transistor that forms a 12N current mirror circuit (6) with the second transistor (02), and its emitter is The ground line (
4) is connected to. Furthermore, the second transistor (Q!)
acts on the input side transistor of the current mirror circuit (6), and the fifth transistor (Q,) acts on the output side of the current mirror circuit (6). It forms a register. On the other hand, the fourth transistor (Q4) serves as the input transistor of the 1:1 current mirror circuit (7), and the sixth transistor (Q,) serves as the output transistor. The terminal Q'7 of this sixth transistor (0,) is connected to the ground line (4) via a resistor (R,).

In the circuit shown in Figure 1, the reference current is the fifth transistor (QS
), and a constant current N times that current is output from the collector of the sixth transistor (Q6). Here, for resistances (R+) to (R,), R++Ih=R
s+R4, R4 child R6. Rs/! It is assumed that the relationship h=N is selected.

Next, the operation will be explained. First and third transistors (fL)
A common bias voltage (V,) is applied to the bases of (0.,), and R + + R 2 = R s +
Since there is a relationship R a , the currents flowing through the resistors (Rz) and (R-) are the same.

Here, this current is assumed to be Nl. 21st Ranjistha (
0■) is N times the size of the fifth transistor (RoS),
And since RS/Rt=N, the resistance (R2) and (R,)
The ratio of the currents flowing in is N:1. Therefore, the current flowing through the collector of the fifth transistor (O,) is .

On the other hand, the sizes of the fourth and sixth transistors (Q.) (Q6) are the same, and R a =R h, so the resistance (
The current flowing through R4) (Rh) is Nl.

Therefore, the current flowing through the collector of the sixth transistor (Q,) is NI. From the above, the transistor (0,
The ratio of the collector currents of ) and (q, ) is 1:N. In this way, although the size of the sixth transistor (Q,) is the same as that of the transistor <0,), it can supply N times as much constant current. Therefore, the parasitic capacitance between the collector of the sixth transistor (Q,) and the ground does not increase.

FIG. 2 shows a specific application example of the constant current circuit shown in FIG. 1, in which the output current I of the fifth transistor (0,) is given to the differential amplifier (8), The output current Nl is applied to a differential amplifier (9). Differential amplifiers (8) and (9) are supplied with alternating current signals from input terminals (10) and (11), respectively, and their output signals are derived to output terminals (12) and (13), respectively. The above constant current circuit is the 6th 1-transistor (0,)
Since the parasitic capacitance (C2) of the fifth transistor (0,) is as small as the parasitic capacitance ffi (CI) of the fifth transistor (0,), there is no risk that the high frequency range of the AC signal will be degraded. Therefore, the frequency characteristics are better than that of the conventional example shown in FIG.

According to the present invention, as described above. , l:N constant current circuit, the parasitic capacitance of the transistor that outputs N times the constant current does not increase. Therefore, even when used as a constant current source in a circuit that processes alternating current signals, the frequency characteristics are not impaired.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a constant current circuit implementing the present invention,
FIG. 2 is a diagram showing an example of its application. FIG. 3 is a circuit diagram of a conventional example, and FIG. 4 is a circuit diagram showing an example of its application. (Q I)' - 1st transistor (Qi) - 1st transistor (Q3) - 1st third transistor ([14) - 4th transistor (Qs) - 1st fifth transistor (g,), 1st sixth transistor ．． (1)・First circuit. (2) (5) Constant voltage source. second circuit,

Claims (1)

[Claims] (1) A first transistor with an emitter area size S and a size NS whose emitter area is N times that of the first transistor.
a first circuit comprising second transistors connected in cascade; a second circuit comprising a third transistor of size NS and a fourth transistor of size S connected in cascade; means for applying the same driving voltage to the two circuits; and the size S forming an output side transistor of a current mirror circuit in which the second transistor is an input side transistor.
and a sixth transistor of the size S forming an output transistor of a current mirror circuit in which the fourth transistor is an input transistor, and a constant current is output from the fifth and sixth transistors. constant current circuit.