JP3140107B2 - Differential amplifier - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a differential amplifier using a folded cascode circuit.
In particular, the present invention relates to a differential amplifier that improves amplification accuracy with a simple circuit configuration.

[0002]

2. Description of the Related Art Conventionally, as a differential amplifier of this type, FIG.
There was the one shown in FIG. A differential amplifier using a folded cascode circuit is used as a high-speed operational amplifier. An advantage of this differential amplifier is that the stagger ratio between the dominant pole and the second pole is wide, so that the negative feedback can be stabilized. Further, since the dominant pole can be set high, an amplifier having good high-frequency characteristics can be realized. Here, the transistor connected to the folded cascode forms an amplifier with a grounded base, which has a low impedance when viewed from the differential amplifying unit, and therefore the pole is sufficiently high to determine the second pole. The voltage gain A _v becomes only acts as a current transmitter at "1", since it is the first-stage differential amplifier with a cascode connection, and thus to convert the input voltage V _i to the current. Therefore, the next stage impedance connected to the output terminal determines the dominant pole.

In the conventional differential amplifier shown in FIG. 2, two positive and negative input signals are input to input terminals 41 and 42 connected to a base terminal, and the emitter terminals are commonly connected to form a differential amplifier 10. and two transistors T _r1, T _r2 of,
Constant current source is connected between the commonly connected emitter terminals of the two transistors T _r1, T _r2 and the ground GND 1
And constant current sources 4 and 5, one _ends of which are connected to the collector terminals of the transistors Tr1 and _Tr2 , respectively, and the other end of which is commonly connected to a power supply V ^+. _r1, respectively to the connection point of the T _r2 is connected to the emitter terminal and the transistor T _r3, T _r4 to form a folded cascode circuit 20, the battery applies a bias voltage to the base terminal of the transistor T _r3, T _r4 6 When, with the collector terminal connected to the collector terminal of the transistor T _r3, T _r4, and an emitter terminal connected to the ground GND in the configuration and a two transistor T _r50, T _r60 which form a current mirror circuit 30 is there.

[0004] Then the output terminal 51 a connection point between the collector terminal of the collector terminal of transistor T _r60 of the transistor T _r4, the output terminal and the ground GND resistor R _L
Is connected.

Next, a conventional differential amplifier based on the above configuration will be described.
Will be described. To two input terminals 41 and 42
Input voltage V of input signal_i ⁺, V_i ^-Is applied and this input
The differential voltage of the force signal is V_iThen, the transistor T_r3No
The current ｛I- (I_E/ 2) -i｝ is supplied
And the transistor T_r4Current to the emitter terminal of
｛I- (I_E/ 2) + i} is supplied. Where I_E
Is the current value of the constant current source 1, I is the current value of the constant current sources 4 and 5,
i is the input voltage V_i ⁺, V_i ^-Of collector current due to
Minutes. Also, the transistor T_r50, T_r60Is Karen
Each collector
・ The emitter-to-emitter current value becomes constant and the resistance R _LThrough the ground
The current 2i of these differences flows through the
Is the output voltage V₀= R_L× 2i.

The relationship between the difference voltage V _{i of the} input signal and the current i is as follows. Where _{V i = V BE1 -V BE2,} V BE1 is the base-emitter voltage of the transistor T _r1, V _BE2 is the base-emitter voltage of the transistor T _r2.

[0007] _{V i = (kT / q)} ln [{(I E / 2) + i} / I s] - (kT / q) ln [{(I E / 2) -i} / I s] = ( kT / q) ln [{( _IE / 2) + i} / { _IE / 2) -i} where k is Boltzmann's constant, T is the absolute temperature, and q is the charge of the electron.

Further, the conventional differential amplifier shown in FIG.
FIG 2 differential amplifiers as well as the transistor T _r1, T _r2 described, is constituted by a constant current source 1, having different configurations of the transistors of the transistor and the current mirror circuit of the folded cascode connection . The constant current source 3 is connected between the ground GND and the output terminal 51 instead of the transistor of the current mirror circuit. The transistors of the folded cascode connected consists of a single transistor T _r3, with the emitter terminal of the transistor T _r3 is connected to the collector terminal of the transistor T _r2, the collector terminal of the transistor T _r3 constant current source 3 and the output terminal 51. A resistor _RL is connected between the output terminal 51 and the ground GND.

As described above, the differential amplifier shown in FIG.
Amplified output can be achieved by simplifying the circuit configuration of the differential amplifier described above.

[0010]

THE INVENTION Problems to be Solved] Since the conventional differential amplifier was configured as described above, the transistor a signal current output from the transistor T _r1, T _r2 in the differential amplifier of FIG. 2, wherein T _{Since the signals} are supplied to _r3 and _Tr4 , respectively, an amplification error occurs due to a transmission error caused by unevenness of the elements of the transistors _Tr3 and _Tr4 .
In particular, this error acts to increase because the increase and decrease of the currents of the two transistors are opposite to each other.

In the differential amplifier shown in FIG.
Although the circuit configuration is simplified and the amplification error is reduced as compared with the case where there are two folded cascode transistors as shown in FIG. 2, there is a problem that the gain is reduced by half. The gains of the differential amplifiers of FIGS. 2 and 3 are compared and calculated as follows.

First, the first-stage differential input impedance Z _i of each differential amplifier is Z _i = 2h _ie . Next, the transmitted signal current i is i = (V _i / Z _i ) · h _fe = (V _i / 2h _ie ) · h _fe . The output voltages V ₀₁ (the output voltage in FIG. 2), V ₀₂
(Output voltage in FIG. 3) is V ₀₁ = 2i · _RL V ₀₂ = i · _RL , and the differential amplifier shown in FIG. 3 has a change in output current due to the input voltage with respect to the differential amplifier shown in FIG. The divided current i is 1 /
It is 2.

The voltage gain A _v1 (gain in FIG. 2) and the voltage gain A _v2 (gain in FIG. 3) are as follows: A _v1 = V ₀₁ / V _i = (2i · _RL ) / ｛(2h _ie / h _fe ) _{· i} = (h fe /} h ie) · R L A v2 = V 02 / V i = (i · R L) / {(2h ie / h fe) · i} = (h fe / 2h ie) · _RL .

Therefore, the voltage gain A _v2 of the differential amplifier shown in FIG. 3 is smaller (（) than the voltage gain A _v1 of the differential amplifier shown in FIG. The present invention has been made to solve the above-described problems, and has as its object to propose a differential amplifier in which an amplification error is reduced as much as possible and a circuit configuration is simplified.

[0015]

A differential amplifier according to the present invention is formed by connecting two emitters in parallel with an emitter terminal connected in common between a power supply having one potential and a power supply having a reference potential. A differential transistor that outputs a signal proportional to a difference between two input signals input to each base terminal, and two transistors connected between the one-potential power supply and each collector terminal of the differential transistor. A current mirror circuit formed with the one transistor as a reference side and the other transistor as a subordinate side, a collector terminal of a subordinate transistor in the current mirror circuit, and one collector in the differential transistor. Folded cascode transformer that is connected between the connection point where the terminal is connected and the output terminal and is formed by a single transistor And register, which is connected to a connection point between the other collector terminal at the collector terminal and the differential transistors of the reference side transistor in the current mirror circuit, in which and a constant current source for supplying a predetermined current.

[0016]

According to the present invention, a differential transistor and a current mirror circuit are connected in series, and a single folded cascode transistor is connected to the connection point of the series connection, so that the first-stage differential amplification of the differential transistor is performed. The output is push-pull-combined by a current mirror circuit, and a bias is applied to the folded cascode transistor by applying a bias from the reference side of the current mirror circuit to a constant current source. And a high gain can be obtained.

[0017]

An embodiment of the present invention will be described below with reference to FIG. FIG. 1 shows a circuit configuration diagram of the present embodiment. In the same figure, in the differential amplifier according to the present embodiment, two positive and negative input signals are input to input terminals 41 and 42 connected to the base terminal similarly to the conventional differential amplifier shown in FIG. configured with two transistors T _r1, T _r2 which are commonly connected to form a differential amplifier 10, current mirror circuit 30, the configuration contents and connection configuration of the constant current source 2 and the folded cascode circuit 20 Make a difference.

The current mirror circuit 30 includes the differential
Transistor T of amplifier 10_r1, T _r2To the collector terminal of
Each collector terminal is connected and the emitter terminal
Power supply V⁺Two transistors T connected to_r5, T_r6so
Formed, the two transistors T_r5, T_r6Each ba
And the common connection point of this base terminal
The collector terminal is connected to one connection point A.
You. Therefore, this current mirror circuit 30
T_r5And the transistor T_r6Is dependent
Operate.

In the folded cascode circuit 20, the emitter terminal is connected to the other connection point B of the collector terminal of the current mirror circuit 30, and the collector terminal is connected to the ground GND via the output terminal 51 and the constant current source 3. The configuration includes a single transistor _Tr3 and a battery 6 for applying a predetermined voltage to the base terminal of the transistor _Tr3 . Note that the battery 6 may be a constant voltage circuit of a predetermined voltage. A resistor _RL is connected between the output terminal 51 and the ground GND.

Further, the constant current source 2 to I _B the current value between one connection point A of the collector terminal and the ground GND is a connection point current mirror circuit 30 and the differential amplifier 10 is connected always against the folded cascode circuit 20 by the operation of the constant current source 2 and the current mirror circuit 30 ( "0" input signal, "positive", regardless of the "negative") by supplying the bias current I _B I have.

Next, the operation of this embodiment based on the above configuration will be described. First, the voltage V _i ⁺ of the input signal to the input terminal 41 connected to the base terminal of the transistor T _r1 is input, the transistor T voltage to the input terminal 42 connected to the base terminal of _r2 V _i ^- the input signal is input is, the differential voltage between the respective input signal and V _i. In this case, the constant current source 1
When the current flowing to the I _E, the transistor T _r1 is
As in the prior art shown in FIG. 2, the current ｛(I _E / 2) +
i} flows, and a current {( _IE / 2) -i} flows through the transistor _Tr2 . The current in the transistor _{T r5 {(I E / 2} ) + i + I B} flows, the transistor T same current flowing through the transistor T _r5 in transistor T _r6 to form a current mirror circuit 30 together with the _r5 {(I _{E / 2) + i + I} B} so that the flows. Therefore, the current flowing from the connection point B to the folded cascode circuit 20 in FIG.
current flowing through _r6 current flowing from _{{(I E / 2) +} i + I B} to the transistor T _r2 so that the flow minus the _{{(I E / 2) -i} }.

The folded cascode flows to the circuit 20 a current _{= (I E / 2) +} i + I B - {(I E / 2) -i} = 2i + I B I B during this formula is the bias current of the transistor T _r3.

Then, the current of the constant current source 3 is changed to a current I _B
If it is set equal to, the current flowing through the resistor _RL is 2i. At this time, the voltage gain A _V has the following value similar to the conventional example shown in FIG.

A _V = (h _fe / h _ie ) · _RL Therefore, this configuration allows the voltage gain A _V to be the same as that of the conventional differential amplifier composed of two folded cascode-connected transistors, and It is possible to reduce an amplification error caused by the transistor.

In the embodiment, the current mirror circuit 30 is the most basic one composed of two transistors, but may be a well-known feedback type using three transistors. Another current mirror circuit may be used.

[0026]

As described above, according to the present invention, a differential transistor and a current mirror circuit are connected in series,
By connecting a single folded cascode transistor to the connection point of the series connection, the first stage differential amplification output of the differential transistor is push-pull-combined by a current mirror circuit, and furthermore, the output is fixed from the reference side of the current mirror circuit. By applying a bias by the current source, a bias current can be supplied to the folded cascode transistor, and the effect of reducing amplification errors and obtaining high gain with a simple circuit is achieved.

[Brief description of the drawings]

FIG. 1 is a circuit configuration diagram according to a differential amplifier of the present invention.

FIG. 2 is a circuit diagram of a conventional differential amplifier.

FIG. 3 is a circuit configuration diagram of another conventional differential amplifier.

[Explanation of symbols]

1,2,3,4,5 ... constant current source _{T r1, T r2, T r50} , T r60 ... NPN type transistor _{T r3, T r4, T r5} , T r6 ... PNP -type transistor 10 ... differential amplifier 20 ... Folded cascode circuit 30 ... Current mirror circuit 41,42 ... Input terminal 51 ... Output terminal

Claims (1)

(57) [Claims] An emitter terminal is connected in common between a power supply having one potential and a power supply having a reference potential, and two transistors are connected in parallel to form two input signals. A differential transistor that outputs a signal proportional to the difference, two transistors are connected between the power supply of one potential and each collector terminal of the differential transistor, and the one transistor is used as a reference side, A current mirror circuit formed by using another transistor as a slave side; and a connection point between the collector terminal of the slave transistor in the current mirror circuit and one collector terminal of the differential transistor, and an output terminal. A folded cascode transistor formed of a single transistor and connected between the current mirror circuit; A constant current source connected to a connection point between a collector terminal of the reference side transistor and the other collector terminal of the differential transistor and supplying a predetermined current.