JP3213975B2 - Push-pull 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 broadband push-pull amplifier, and more particularly to a push-pull circuit comprising an NPN transistor and a PNP transistor provided at an output stage for driving a capacitive load to a wide band. It is suitable.

[0002]

2. Description of the Related Art In order to drive a capacitive load connected to an output side, an amplifier having a push-pull circuit including an NPN transistor and a PNP transistor at an output stage is sometimes used. Although the circuit configuration of such a push-pull amplifier is various, for example, a circuit configuration as shown in FIG. 5 can be considered.

In the circuit shown in FIG. 5, a transistor Q2 and a transistor Q3 are provided at an output stage with their emitters commonly connected.
The output is derived from the connection point No. 3. In this case, an NPN transistor is used as the transistor Q2, and a PNP transistor is used as the transistor Q3.

On the other hand, the input stage is provided with a transistor Q1 for driving the transistor Q2. An NPN transistor is used as the transistor Q1, and the transistors Q1 and Q2 constitute a Darlington connection circuit. A first resistor R is connected between the emitter of the driving transistor Q1 and ground.
1, a second resistor R2 and a constant current source 1 are connected in series.

Further, a fourth transistor Q4 composed of an NPN transistor is connected between the base of the transistor Q2 and the base of the transistor Q3. The fourth transistor Q4, together with the first and second resistors R1 and R2, constitutes a bias circuit for the transistors Q2 and Q3, and determines the idling current of these transistors Q2 and Q3.

[0006]

In the circuit of FIG. 5 configured as described above, the transistor Q has an arbitrary frequency.
The value of the current I ₀ flowing through the constant current source 1 is determined so that the load 3 can drive the load capacitance C. However, the transition frequency f _T of the PNP transistor is lower than that of the NPN transistor.
The current amplification factor h _FE of the transistor Q3 becomes low. Therefore, in order to extend the frequency characteristic to the desired frequency, the current I ₀ of the constant current source 1 has to be further increased.

However, the emitter current of the transistor Q1 becomes the constant current I ₀ when there is no signal. Therefore, the more the current I ₀ is increased, the more the first transistor Q1
And the voltage drop due to the input impedance _RL and the base current flowing through the input impedance _RL , I ₀ / {h _FEN1 +1} becomes large, so that the dynamic range is disadvantageously reduced. However, h _FEN1 is the transistor Q1
3 shows the current amplification factor of FIG. In order to solve such a problem, it is conceivable to provide a transistor for driving the first transistor Q1. However, in this case, the dynamic range is reduced in the same manner as described above, so that the constant current I ₀ cannot be steadily increased. The present invention has been made in view of the above problems, and
It is an object of the present invention to achieve both improvement in characteristics and securing a necessary dynamic range.

[0008]

Means for Solving the Problems] flop Sshupuruanpu of the present invention, a second transistor formed of NPN transistors, and an output push-pull circuit comprising a third transistor formed of a PNP transistor, said second transistor A first transistor provided in the input stage for driving and constituting a Darlington connection circuit together with the second transistor, interposed between an emitter of the first transistor and a current source;
A bias circuit that supplies a bias current to the base of the second transistor and the base of the third transistor, and is connected between the base of the third transistor and a power supply via the emitter of the first transistor A fifth transistor for allowing a current not to flow to the current source, and the bias circuit includes a first transistor .
A series connection between the transistor and the current source.
The first and second resistors and the base are connected to the first and second resistors.
And the connection point of the second resistor and the base of the fifth transistor.
And a fourth transistor connected to the source.

[0009]

By increasing the current flowing through the current source without passing through the first transistor, the voltage drop due to the input impedance of the first transistor does not increase even if the current of the current source is increased. And then
It is possible to improve the frequency characteristics while preventing the dynamic range from being lost.

[0010]

FIG. 1 is a circuit diagram of a broadband push-pull amplifier showing one embodiment of the present invention. In the broadband push-pull amplifier of this embodiment, first, an input signal is input to a signal source V _IN . In the circuit of FIG. 1, R _L is an equivalent input resistance, and considering a circuit configuration in an IC as shown in the configuration diagram of FIG. 2, an amplifier 3 connected in front of the amplifier 2 of the present embodiment Output impedance.

In the case of the circuit of this embodiment, the resistors R1, R
The idling current of the transistor Q2 and the transistor Q3 is determined by a bias circuit including the transistor Q2 and the transistor Q4. Note that the third transistor Q
The capacitance C connected to the collector of No. 3 is a total of the pad capacitance in the IC chip and the pin capacitance of the IC package. The first transistor Q1 is provided for driving the second transistor Q2. In the case of the present embodiment, the transistors Q1 and Q2 form a Darlington connection circuit in the same manner as the above-described conventional circuit. ing.

In the wide-band push-pull amplifier of this embodiment configured as described above, the current amount of the constant current source 1 is obtained as follows. That is, the output amplitude is V
_PP , the frequency to be obtained is f _max, and the rise and fall times at that time are t _min ,

(Equation 1)

(Equation 2) Holds.

When the maximum current I _max is obtained from “Equation 1” and “Equation 2”,

(Equation 3) It becomes as shown in. That is, when the output amplitude V _PP , the maximum frequency f _max , and the load capacitance to be obtained as outputs in the circuit configuration of FIG.
The emitter current is required by the maximum current I _max in "number 3".

Therefore, assuming that the current amplification factor of the NPN transistor is h _FEP , the constant current source 1
Of the current I is required at least by the amount determined by “Equation 4”.

(Equation 4) "Number 4" As is apparent from _T ransition frequency f _T of the PNP transistor, as compared to _T ransition frequency f _T of the NPN transistor, the maximum frequency f _max the higher current amplification factor h _FEP decreases, a current source Even more current I ₀ is needed.

That is, for example, when the current I ₁₀₀ required at ₁₀₀ MHz is compared with the current I ₂₀₀ required at 200 MHz, the current I ₁₀₀ in the ₁₀₀ MHz band is I ₁₀₀ = (100 · Vm) / (10+
1). Also, the 200 MHz current I _200
200 = (200 · Vm) / (5 + 1) Therefore, current I ₂₀₀ / current I ₁₀₀ ≒ 3.7, and in the high frequency band, if the frequency doubles, the current will need to be about four times, and the higher the frequency, the more current is required. You can see that.

As described above, conventionally, if the current I ₀ flowing through the current source is increased, the emitter current of the first transistor Q1 is also increased, which is the product of the resistor _RL and I / (h _FEN +1). , R _L / I / (h _FEN +1), which reduces the dynamic range of the amplifier. Also, the current amplification factor h _FE of the NPN transistor
There is a problem that the dynamic range also changes due to the variation in the dynamic range, and the variation in the dynamic range becomes large. If the first transistor Q1 is Darlington-connected to solve this problem, there is a disadvantage that the dynamic range is further reduced.

In this embodiment, in order to improve the frequency characteristics without such inconvenience, as shown in the circuit of FIG. 1, the power supply is connected between the power supply Vcc and the base of the transistor Q3.
A fifth transistor Q5 is connected, and this transistor Q5
5 and the base of the fourth transistor Q4 are commonly connected.

Thus, the fifth transistor Q5
Is connected, the current flowing through the transistor Q5 becomes n
Assuming that I ₁ , the current I flowing through the constant current source 1 is (I ₀
+ NI ₁ ), and the current I flowing to the constant current source 1 can be increased without increasing the current I ₀ flowing to the emitter of the first transistor Q1. Therefore, when the input signal falls, the maximum (I ₀ + nI ₁ ) · (h _FEP +
The load capacitance C can be driven by the current 1).

Therefore, it becomes possible to drive the load capacity more by the current of nI ₁ · (h _FEP +1) than the conventional circuit shown in FIG. For this reason,
It becomes a high frequency band and the current amplification factor h _{FE of the} PNP transistor
, A high f characteristic can be obtained. That is, the wideband push-pull amplifier of the present embodiment can extend the frequency characteristic to the high frequency band without sacrificing the dynamic range. FIG. 3 shows the currents at the emitters of the transistors Q2 and Q3 and the fifth transistor Q5 when the pulse signal P _IN is input from the input terminal and the pulse signal P _OUT is output from the output terminal.

In order to increase the current I flowing to the constant current source 1 without increasing the current I ₀ flowing to the emitter of the first transistor Q1, as shown in the circuit diagram of FIG. A method of connecting the sixth transistor Q6 in parallel with the transistor Q1 is also conceivable. However, the PNP transistor has a collector-to-ground capacitance C _CS
Therefore, there is an inconvenience that the f characteristic is deteriorated by such a configuration. Also, the PNP transistor as described above because of the low transition frequency f _T, becomes a high-frequency band, the current amplification factor h _FE would quickly drop. As a result, the collector current of the transistor Q6 decreases, and the emitter current of the transistor Q1 increases.
There is a disadvantage that DC loss occurs as in the conventional circuit. Therefore, it is not possible to improve both the f characteristic and the dynamic range as well as the circuit of the present embodiment shown in FIG.

[0021]

As described above, the present invention is provided at the input stage to drive the output push-pull circuit comprising the second transistor and the third transistor provided at the output stage. Since the current that does not pass through the first transistor flows to the current source, the first
The amount of current flowing to the current source can be increased without increasing the amount of current flowing to the transistor, and the ability of the push-pull circuit to drive a load capacitance can be improved. As a result, the f characteristic can be improved without increasing the voltage drop due to the input impedance of the first transistor, and the f characteristic can be improved without sacrificing the dynamic range.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a broadband push-pull amplifier showing one embodiment of the present invention.

FIG. 2 is a schematic configuration diagram in an IC provided with a wideband push-pull amplifier of the present embodiment.

FIG. 3 is a waveform diagram showing a waveform of a current flowing through each element.

FIG. 4 is a circuit diagram showing a modification of the embodiment.

FIG. 5 is a circuit diagram showing an example of a conventional wideband push-pull amplifier.

[Explanation of symbols]

Reference Signs List 1 constant current source 2 amplifier of embodiment 3 amplifier connected in front stage C load capacitance Q1 first transistor Q2 second transistor Q3 third transistor Q4 fourth transistor Q5 fifth transistor I current flowing through current source I ₀ Current flowing through the first transistor nI ₁ Current flowing through the fifth transistor

Claims (1)

(57) [Claims] An output push-pull circuit including a second transistor including an NPN transistor and a third transistor including a PNP transistor;
A first transistor which is provided in the input stage for driving the transistor of the first embodiment and forms a Darlington connection circuit together with the second transistor, and is interposed between the emitter of the first transistor and a current source. A bias circuit for supplying a bias current to the base of the second transistor and the base of the third transistor; a bias circuit connected between the base of the third transistor and a power supply; current which does not pass through the emitter comprises a fifth transistor to flow to the current source, the bias circuit, said collector and said first transistor
A first and a second arrangement connected in series with the flow source;
A resistor and a base are connected to the first and second resistors.
Connected to the connection point and the base of the fifth transistor.
A push-pull amplifier comprising a fourth transistor .