JPS6117621Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a differential amplifier, and more particularly to a distortion-free differential amplifier using bipolar transistors.

When an amplifier is configured with bipolar transistors, the difference between its collector current I _c and base-emitter voltage V _BE is I _c = I _s {exp (qV _BE /KT)-1} ...(1) The relational expression is established. Here, I _s is the reverse saturation current, q is the electronic charge, K is Boltzmann's constant, and T
respectively indicate the absolute temperature of the junction. As described above, since the input characteristics of a bipolar transistor have non-linearity, the collector output current waveform has a large distortion with respect to the input voltage waveform.

In order to eliminate such distortion, for example, a method of inserting an emitter resistor and applying so-called negative current feedback is adopted, but not only is it not possible to completely eliminate distortion, but if the amount of feedback is increased, the circuit gain decreases. This is also undesirable because it also induces circuit instability. Figure 1 shows an example of a commonly used differential amplifier circuit.
The emitters of the differential pair of PNP transistors Q ₃ and Q ₄ are commonly connected via current feedback emitter resistors R ₃ and R ₄ , respectively, and are supplied with a bias current by a current source Io. The base of the transistor _Q4 is given a reference potential, that is, the ground potential, and the input signal e _i is applied to the base of the transistor _Q3 , and the collector resistances _R1 ,
The configuration is such that a differential inverted output is derived from _R2 .

In this differential amplifier, considering the relationship between only AC components excluding DC bias voltage and current, if the AC components of the base-emitter voltages of transistors Q ₃ and Q ₄ are v _he3 and v _he4 , then the input voltage The relationship between v _i and emitter currents i _e3 and i _e4 is as follows: e _i =v _he3 +R ₃・i _e3 +R ₄・i _e4 −v _he4
...(2) becomes. Further, the relationship between the voltage v _ee between both emitters and the positive phase output v _p2 is expressed as v _p2 =R ₂ /R ₃ +R ₄ ·v _ee (3).

Now, considering the case where the input signal e _i changes in the positive direction (increasing direction), the transistor
The emitter current (≒collector current) of _Q3 decreases, and the emitter current (≒collector current) of transistor _Q4 increases. As a result, the fluctuation range ΔV _BE3 , ΔV BE3 of the base-emitter voltage of each transistor
As is clear from the characteristic curve of equation (1) (not particularly shown since it is well known), the relationship of V _BE4 is |ΔV _BE3 |>|ΔV _BE4 (4). Therefore, it can be seen that v _he3 - v _he4 in formula (2) becomes v _he3 - v _he4 >O (5). Therefore, if the input voltage e _i increases, in equation (2), the voltage between both emitters v _ee = R ₃
As the rate of increase in i _e1 +R ₄ ·i _l2 decreases, the rate of increase in output voltage v _p also decreases from equation (3).
On the other hand, the same holds true when the input voltage e _i changes in the negative direction. Therefore, the output waveform v _p for the sinusoidal input waveform e _i has a shape in which the upper and lower peaks are collapsed, as shown in FIG. 2, and distortion occurs.

An object of the present invention is to provide a differential amplifier that can completely eliminate output distortion due to nonlinearity of input/output characteristics in bipolar transistors with a simple configuration.

A differential amplifier according to the present invention includes first and second differential transistors of an emitter follower type, each of which has a first power supply supplied to each collector and a differential input signal applied to each base;
and third and fourth differential transistors of opposite conductivity type to the first and second transistors, which are connected to operate differentially with each other using the emitter follower output of the second transistor as a base input, respectively;
a first current supply means provided between the emitter of the first transistor and the collector of the third transistor and a second power supply to supply a constant ratio of current to the first and third transistors; a second current supply means provided between the emitter of the second transistor and the collector of the fourth transistor and the second power supply in order to supply a constant ratio of current to the second and fourth transistors; In this configuration, the output is derived in response to a change in the current flowing through at least one of the third and fourth transistors.

The present invention will be explained below with reference to the drawings.

FIG. 3 is a circuit diagram showing an embodiment of the present invention, and parts equivalent to those in FIG. 1 are designated by the same reference numerals. The emitter follower outputs of NPN transistors Q ₁ and Q ₂ having an emitter follower configuration are applied to the bases of PNP transistors Q ₃ and Q ₄ , respectively, which have a differential connection configuration and whose emitters are commonly connected through emitter resistors R ₃ and R ₄ , respectively. has been done. transistor
The bases of Q ₁ and Q ₂ serve as differential input terminals, and in this example, the input signal e _i is applied to the base of transistor Q ₁ , and the base of transistor Q ₂ is given a ground potential. There is.

A current mirror circuit _{1 consisting of a transistor Q 5 ,} a diode D ₁ and resistors R ₅ and R ₆ is provided to supply a constant ratio of current to these transistors Q 1 and Q ₃ . Also transistor
A current mirror circuit _{2 consisting of a transistor Q 6 ,} a diode D ₂ and resistors R ₇ and R ₈ is provided to supply a constant ratio of current to Q 2 and Q ₄ . still,
Both diodes D ₁ and D ₂ can be replaced with diode-connected transistors.

In such a configuration, if the relationship between the input voltage e _i and the emitter currents i _e3 and i _e4 of the transistors Q ₃ and Q ₄ is similarly considered for only the alternating current component, the following equation is obtained.

e _i =-v _he1 +v _he3 +i _e3・R ₃ +i _e4・R ₄ −v _he4 +v _he2 ...(6) Here, the resistance of current mirror circuits 1 and 2
Let us consider a case in which the input voltage e _i changes in the positive direction, assuming that R ₅ , R ₆ and R ₇ , R ₈ are made equal and the current mirror ratio is set to 1. In this case, the collector current of transistor Q ₃ operates in the decreasing direction, and the collector current of transistor Q ₄ operates in the increasing direction, so the fluctuation range ΔV _BE of the base-emitter voltage of each transistor is ΔV _BE1 = ΔV _BE3
, and both increase, and ΔV _BE2 =ΔV _BE4 , and both decrease. Therefore, v _he1 and v _he in equation (6)
₃ and v _he2 and v _he4 cancel each other out, and equation (6) becomes e _i = i _e3・R ₃ + i _e4・R ₄ = v _ee ……(7), and the input voltage e _i There is a proportional relationship with the voltage v _ee between both emitters, and there is a one-to-one correspondence. And since the positive phase output is V _p2 = R ₈ / R ₃ + R ₄ · v _ee ...(8), the relationship between the input and output is a proportional relationship that does not include the v _he component at all, and is null. A distorted output will be obtained.

Similarly, when the input changes in the negative direction, v _he of each transistor cancels each other out, resulting in a distortion-free output, and it is clear that the negative phase output v _p1 also becomes distortion-free. .

In the above, the current mirror ratios in the current mirror circuits 1 and 2 were both set to 1, but a case will be considered in which they are both set to a constant value α.
Since the relationship between transistor V _BE and I _c is shown by equation (1), the difference in V _BE between transistors Q ₁ and Q ₃ is
V _BE3 −V _BE1 =KT/q {ln(I _c3 /I _s3 +1)−ln(I _c1 /I
_s1 +1)}...(9) Here, the junction temperatures are assumed to be the same. Then, it can be considered that I _c1 /I _s1 ≫1, I _c3 /I _s3 ≫, and if I _c3 /I _c1 = α (current mirror ratio), equation (9) becomes V _BE3 −V _BE1 ≒KT/ It is expressed as qln(1/α)...(10). However, I _s1 ≒ I _s3 . Similarly, the relationship expressed by equation (10) also holds true for the difference in V _BE between transistors Q ₂ and Q ₄ , and all of them exhibit constant values. Therefore, v _he in equation (6)
₃ -v _he1 and v _he2 -v _he4 are both constant, and even if the current mirror ratio is set to a constant α (excluding 1), the output is independent of the v _he of each transistor and has no distortion. It turns out that it will happen.

FIG. 4 is a circuit diagram of another embodiment of the present invention, in which parts equivalent to those in FIG. 3 are designated by the same reference numerals.
In this example, the emitter resistance is shared and shown as R _E , and two current sources I _p1 and I _p2 are used to supply the emitters of each transistor Q ₃ and Q ₄ , respectively. The configuration is the same as that shown in FIG. 3, and a description thereof will be omitted.

The method for deriving the output is not limited to the example of using the voltage across the resistor of a current mirror circuit.Since the ratio of current flowing through transistors Q ₂ and Q ₄ and transistors Q ₁ and Q ₃ is always in a constant relationship with each other, It is also possible to insert a resistor into the current path and derive the voltage across the resistor as an output.

Furthermore, although a current mirror circuit is used as the current supply means, it is obvious that a circuit configuration having an equivalent function can be used. It goes without saying that the conductivity types of each transistor may be opposite conductivity types.

As described above, according to the present invention, distortion due to the input/output characteristics of transistors can be completely removed without feedback, and a stable differential amplifier can be realized.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a conventional differential amplifier, FIG. 2 is a diagram showing input and output waveforms of the circuit of FIG. 1, and FIGS. 3 and 4 are circuit diagrams showing embodiments of the present invention, respectively. . Explanation of symbols of main parts, Q ₁ , Q ₂ ... NPN type emitter follower transistor, Q ₃ , Q ₄ ... PNP
type differential transistor, 1, 2...current mirror circuit.

Claims (1)

[Scope of utility model registration request] first and second differential transistors of an emitter follower type, each of which has a first power supply supplied to its collector and a differential input signal applied to each base; and emitter follower outputs of the first and second transistors. third and fourth transistors of opposite conductivity type to the first and second transistors, which are connected to operate differentially with each other and have base inputs as base inputs, respectively.
a differential transistor, and a second power supply provided between the emitter of the first transistor, the collector of the third transistor, and a second power supply to supply a constant ratio of current to the first and third transistors. 1
and a current supply means provided between the emitter of the second transistor and the collector of the fourth transistor and the second power source to supply a constant ratio of current to the second and fourth transistors. Second
current supply means, and is configured to derive an output in response to a change in the current flowing through at least one of the third and fourth transistors.