JPH10335944A - Power circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the sound quality of an audio amplifier by improving the distortion factor of a switching type power circuit used for the audio amplifier, etc. SOLUTION: In this power circuit which supplies a supply voltage Vcc to an amplifier circuit 11 that outputs input signals after amplification and supplies either one of a first supply voltage VccH or a second supply voltage VccL which is lower than the first supply voltage VccH to the circuit 11 by selecting them in matching with the fluctuation of the output voltage Vout in the circuit 11, the supply voltage Vcc is not switched to the higher supply voltage VccH immediately, but the second supply voltage VccL is maintained for a fixed period of time, when the supply voltage Vcc is switched to the high voltage VccH.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply circuit, and more particularly, to a so-called switching type power supply circuit used for an audio amplifier or the like, which switches a power supply voltage in accordance with the size of an audio signal to achieve high efficiency. The aim is to improve.

[0002]

2. Description of the Related Art A conventional switching type power supply circuit will be described. This circuit includes, as shown in FIG. 4, power transistors Q1 to Q4 at the final output stage of the audio amplifier 1,
In accordance with the change of the output voltage Vout of the amplifier 1, the high voltage Vcc
H or a low voltage VccL is supplied as a power supply voltage Vcc.

As shown in FIG. 4, the transistor at the final output stage of the amplifier 1 includes Darlington-connected transistors Q1 and Q2 and Darlington-connected transistors Q3 and Q4, and the transistors Q2 and Q4 are connected to each other. It is configured by push-pull connection.
As shown in FIG. 4, the power supply circuit includes a power transistor M1, which is a MOSFET, resistors R1 to R8, a Zener diode ZD1, a driver transistor Q5, and diodes D1 to D3.

The source of the power transistor M1 is connected to the high voltage VccH, and the gate is also connected to the high voltage VccH via the resistor R1. Also, the gate is simultaneously connected to the resistor R
2 is connected to the collector of the driver transistor Q5. The drain is connected to the collectors of the transistors Q1 and Q2 at the final output stage of the amplifier, and at the same time is connected to the low voltage VccL via the diode D1.

The base and the emitter of driver transistor Q5 are connected by a resistor R3, and the emitter is grounded via a resistor R4. A diode D3 and a resistor R8 are connected in series to the base of the driver transistor Q5.
2, a resistor R7 is connected in parallel with the diode D3 and the resistor R8.

[0006] One end of the resistor R5 is connected between the diode D2 and the resistor R7, and the other end is connected to the drain of the power transistor M1. Similarly, one end of the resistor R6 is connected between the diode D3 and the resistor R8, and the other end is connected to the drain of the power transistor M1. Further, a Zener diode ZD1 is connected between the low voltage VccL and the emitter of the driver transistor Q5.

According to this circuit, the output voltage V of the amplifier 1 is
out is constantly input to the power supply circuit from the resistors R7 and R8. When the output voltage Vout is lower than the predetermined reference voltage Vref, the base potential of the driver transistor Q5 input via the resistor R7 and the diode D2 does not increase so much that VBE of the driver transistor Q5 is turned on. Since the voltage does not rise until reaching the voltage, the driver transistor Q5 is off.

Then, since the collector current of driver transistor Q5 does not flow, power transistor M1 is also turned off.
F, the transistors Q1 and Q2 of the final output stage of the amplifier 1
Has a low voltage VccL via a diode D1.
Are supplied as the power supply voltage Vcc. Also, the output voltage Vou
When t is higher than the predetermined reference voltage Vref, the base potential of the driver transistor Q5 input via the resistor R7 and the diode D2 rises and exceeds the voltage at which the VBE of the driver transistor Q5 is turned on. The transistor Q5 turns on.

Then, since the collector current of driver transistor Q5 flows, power transistor M1 is also turned on.
The high voltage VccH is supplied to the collectors of the transistors Q1 and Q2 at the final output stage of the amplifier 1 via the power transistor M1. As described above, in the above power supply circuit, as shown in FIG. 5, when the output voltage exceeds the reference voltage Vref, the power supply voltage Vcc is changed from the low voltage VccL to the high voltage VccL.
When the output voltage falls below the reference voltage Vref, the power supply voltage Vcc changes from the high voltage VccH to the low voltage VccL.
Switch to

As described above, by switching the power supply voltage Vcc according to the magnitude of the output voltage Vout, the loss of power consumption for supplying the amplifier with a high voltage capable of taking out the maximum output at all times is reduced, and the efficiency is improved. There are advantages.
The driver transistor Q5 in the above circuit is
A device has been devised to operate as a so-called hysteresis comparator.

That is, in a general switching type power supply circuit, when the output voltage Vout fluctuates above and below the reference voltage Vref, the driver transistor Q5 is placed in an intermediate state between the ON state and the OFF state. This tends to cause a disadvantage that the operation of the power supply circuit becomes unstable. Therefore, in order to suppress this inconvenience, in this circuit, at the moment when the power transistor M1 is turned on, a constant voltage is added to the output voltage Vout to suppress the instability of the operation near the reference voltage Vref. doing.

In the above circuit, in the case of ch1, a resistor R5 is connected in series with a resistor R7 for inputting the output voltage Vout as shown in FIG. Accordingly, when the power transistor M1 is turned on, the potential Va at the point A in FIG. 4 rises by a voltage determined by the bleeder ratio of the resistors R5 and R7. The resistor R5 (68 kΩ) is connected to R7 (4.
7 kΩ), and the rise is very small, so the potential Va rises from the output voltage Vout by this minute voltage.

Therefore, the output voltage Vout is equal to the reference voltage Vref.
The power transistor M
When 1 is turned on, the output voltage Vout always exceeds the reference voltage Vref by a minute voltage, so that the driver transistor Q5 keeps the ON state during that time, and as in a general switching type power supply circuit, The inconvenience that the operation becomes unstable due to an intermediate state between the ON state and the OFF state can be avoided.

[0014]

In the above conventional circuit,
By turning on the driver transistor Q5 with a certain margin applied to the output voltage Vout when the power transistor M1 is turned on, the problem that the operation becomes unstable when a signal changes near the reference voltage Vref is solved. However, the following problems have occurred.

That is, the output voltage Vout is 20 kHz
When a high frequency continuous wave is output, the driver transistor Q5 and the power transistor M1 repeat the ON / OFF operation at a considerably high frequency as shown in FIG. 6 in the above circuit, but the output voltage Vout changes. If the power supply voltage Vcc is steep, the supply of the power supply voltage Vcc cannot follow this change, and the output of the amplifier 1 will be distorted, causing a problem that the distortion factor will be deteriorated.

[0016]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned drawbacks. As shown in FIG. 1, a power supply voltage is supplied to an amplifier circuit for amplifying and outputting an input signal. And a second power supply voltage higher than the first power supply voltage according to a change in the output voltage of the amplifier circuit, and supplies the selected power supply voltage to the amplifier circuit. , MOSFET power transistors,
A fifth diode, a Zener diode, a driver transistor, a buffer transistor, a first diode, a second diode serving as an input part of the output voltage, a third diode serving as an input part of the output voltage, and a capacitor; A source of the power transistor is connected to the second power supply voltage, a gate of the power transistor is connected to the second power supply voltage via the first resistor, and a drain of the power transistor is connected to the amplifier circuit. Connected to the first power supply voltage via the first diode, a gate of the power transistor connected to a collector of the driver transistor via a second resistor, and a base and an emitter of the driver transistor. Is
A third resistor, an emitter of the driver transistor connected to an emitter of a buffer transistor, a base of the driver transistor, a fifth resistor and a third diode connected in series, The second diode is connected in parallel with the buffer, one terminal of the capacitor is connected to the emitter of the driver transistor, and the other terminal is connected to the base of the driver transistor via the fifth resistor. The transistor is grounded, the base is grounded via a fourth resistor, and the zener diode is connected between the first power supply voltage and the base of the buffer transistor. A power supply voltage is supplied to a power supply circuit that amplifies an input signal and an amplifier circuit that amplifies and outputs an input signal. A power supply circuit that selects one of a voltage and a second power supply voltage higher than the first power supply voltage in accordance with a change in an output voltage of the amplifier circuit and supplies the selected power supply to the amplifier circuit; Transistors, first to fifth
Resistance, Zener diode, driver transistor,
The power transistor includes a first diode, a second diode serving as an input part of the output voltage, a third diode serving as an input part of the output voltage, and first and second capacitors, and a source of the power transistor is the second diode. 2, the gate of which is connected to the second power supply voltage via a first resistor, and the gate of which is connected to the collector of a driver transistor via a second resistor. The drain is connected to the amplifier circuit, and at the same time is connected to the first power supply voltage via a first diode. The base and emitter of the driver transistor are connected by the third resistor, and the emitter is connected to the second resistor. And a grounded fourth resistor is connected in parallel with the second capacitor, and a fifth transistor is connected to the base of the driver transistor. A third diode is connected in series, the second diode is connected in parallel with the third diode, and one terminal of the first capacitor is connected to the third diode and the fifth resistor. And the other terminal is grounded, and the Zener diode is connected between the first power supply voltage and the emitter of the driver transistor. This is to solve the above-mentioned problem.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION

(1) First Embodiment A power supply circuit for an audio amplifier according to a first embodiment of the present invention will be described below with reference to the drawings. As shown in FIG. 1, the power supply circuit includes first to fourth power transistors Q in the final output stage of the audio amplifier 11.
A circuit for supplying any one of the high voltage VccH and the low voltage VccL to the power supply voltage Vcc from 11 to Q14 in accordance with the change of the output voltage Vout of the amplifier 11. The output stage of this amplifier 11 is a Darlington-connected transistor Q1.
1, Q12 and a transistor Q connected in Darlington
13 and Q14, and the transistors Q12 and Q14 are configured by push-pull connection. The low voltage VccL is an example of a first power supply voltage, and the high voltage VccL
H is an example of the second voltage.

As shown in FIG. 1, this power supply circuit includes a power transistor M11, first to fifth resistors R11 to R1.
5, a zener diode ZD11, a driver transistor Q15, a buffer transistor Q16, and first to third diodes D11 to D13. The source of the power transistor M11 is connected to the high voltage VccH, and the gate is also connected to the high voltage VccH via the first resistor R11. Further, this gate is connected to the collector of the driver transistor Q15 via the second resistor R12 at the same time. The drain is connected to the collectors of transistors Q11 and Q12 at the final output stage of the amplifier, and
Is connected to the low voltage VccL via the diode D11.

The base and the emitter of the driver transistor Q15 are connected by a third resistor R13, and the emitter is connected to the emitter of the buffer transistor Q16. The resistor R15 and the diode D13 are connected in series to the base of the driver transistor Q15, and the second diode D12 is connected in parallel with the third diode D13.

Further, one end of the capacitor C1 is connected to the emitter of the driver transistor Q15, and the other end is connected to the base of the driver transistor Q15 via the fifth resistor R15. The collector of the buffer transistor Q16 is grounded, and its base is grounded via a fourth resistor R14. Further, a Zener diode ZD11 is connected between the low voltage VccL and the base of the buffer transistor Q16.

The operation of the above circuit will be described below.
The output voltage Vout of the amplifier 11 is always input to the above power supply circuit via the second and third diodes D12 and D13. The second diode D12 serves as an input section for the output voltage of ch1, and the third diode D13 serves as an input section for the output voltage of ch2. In the circuit shown in FIG. 1, only the amplifier 11 corresponding to ch1 is shown.
Only the case will be described.

With respect to ch1, when the output voltage Vout of the amplifier 11 is lower than the predetermined reference voltage Vref, the base potential of the driver transistor Q15 does not sufficiently rise, and reaches a voltage at which VBE of the driver transistor Q15 turns on. Driver transistor Q15 is OFF
doing. Then, since the collector current of driver transistor Q15 does not flow, power transistor M11 is also turned off.
FF and the transistor Q1 at the final output stage of the amplifier 11
1, a collector of Q12 is supplied with a low voltage VccL as a power supply voltage Vcc via a first diode D11.

The output voltage Vout is equal to a predetermined reference voltage Vout.
When the voltage is higher than ref, the base potential of the driver transistor Q15 input via the fifth resistor R15 and the second diode D12 rises, and the VBE of the driver transistor Q15 becomes a high voltage. Q15 turns ON. Then, the driver transistor Q1
5 flows through the power transistor M
11 is also turned on, and the transistor Q of the final output stage of the amplifier 1 is turned on.
The power transistor M1 is connected to the collectors of Q11 and Q12.
1 is supplied with a high voltage VccH.

In the circuit of this embodiment, a buffer transistor Q16 is connected to the emitter of the driver transistor Q15 as shown in FIG. 1. This is because the buffer transistor Q16 is turned on again immediately after the driver transistor Q15 is turned on. This is to prevent the potential from rising and turning off again. As described above, in the above circuit, the reference voltage Vref is changed to the output voltage Vou
When t exceeds t, the power supply voltage Vcc becomes the high voltage VccH, and when the output voltage falls below the reference voltage Vref, the power supply voltage Vcc becomes the low voltage VccL, which is the same as the conventional circuit. different.

In the above circuit, which is a feature of the present embodiment, when the output voltage Vout falls below the reference voltage Vref again after the output voltage Vout exceeds the reference voltage Vref and the power supply voltage Vcc becomes the high voltage VccH. The operation will be described below. In this case, for example, in the case of ch1, even if the output voltage Vout input from the second diode D12 decreases, the capacitor C1 is charged when the voltage becomes high VccH.
The base potential of the driver transistor Q15 does not immediately decrease until the electric charge charged to is discharged. Until the base potential drops sufficiently, the base potential of driver transistor Q15 is determined by the time constant of fifth resistor R15 and capacitor C1.

For this reason, the emitter potential of driver transistor Q15 does not immediately decrease and driver transistor Q15 maintains the ON state for a while, whereby power transistor M11 maintains the ON state for a while. . Therefore, the power supply voltage Vcc does not immediately switch from the high voltage VccH to the low voltage VccL, but maintains the state of the high voltage VccH for a certain period of time.

As a result, as shown in FIG. 2, even when the high-frequency output voltage Vout is output, the power supply voltage Vcc does not immediately switch from the high voltage VccH to the low voltage VccL. Neither M11 nor the driver transistor Q15 repeats the ON / OFF operation at high speed, and the power supply voltage Vcc becomes the high voltage VccH.
It will be maintained as it is.

Therefore, no matter how high-frequency output voltage Vout is output, the situation where the change in the power supply voltage Vcc cannot follow the change in the high-frequency output voltage Vout does not occur. It is possible to suppress distortion. As a result, the distortion factor can be improved and the sound quality of the amplifier can be improved as compared with a conventional circuit that has performed a hysteresis operation.

Of course, the output voltage Vout is
It is needless to say that switching the power supply voltage Vcc in accordance with the magnitude of the power supply voltage reduces the loss of power consumption for supplying the amplifier with a high voltage that can always take the maximum output, thereby improving the efficiency. (2) Second Embodiment A power supply circuit for an audio amplifier according to a second embodiment of the present invention will be described below with reference to the drawings. Note that the description of items common to the first embodiment will be omitted to avoid duplication.

As shown in FIG. 3, the power supply circuit supplies high voltage VccH and low voltage VccL to power transistors Q21 to Q24 at the final output stage of the audio amplifier 21 in accordance with changes in the output voltage Vout of the amplifier 21. Is supplied as the power supply voltage Vcc. This amplifier 21
Output stage is a Darlington-connected transistor Q2
1, Q22 and a transistor Q connected in Darlington
23 and Q24, and the transistors Q22 and Q24 are configured by push-pull connection.

As shown in FIG. 3, the power supply circuit comprises a power transistor M21, first to fifth resistors R21 to R2.
5, a zener diode ZD12, a driver transistor Q25, and first to third diodes D21 to D23. The source of the power transistor M21 is a high voltage Vcc
H and the gate is also connected to the high voltage VccH via a first resistor R21. Also, this gate is simultaneously connected to the driver transistor Q2 via the second resistor R22.
5 collectors. The drain of the power transistor M21 is connected to the collectors of the transistors Q21 and Q22 at the final output stage of the amplifier 21, and is also connected to the low voltage VccL via the first diode D21.

The base and the emitter of the driver transistor Q25 are connected by a resistor R23, and the emitter is grounded via a capacitor C12. A grounded fourth resistor R24 is connected in parallel with the capacitor C12. . A fifth resistor R25 and a third diode D23 are connected in series to the base of the driver transistor Q25, and a second diode D22 is connected in parallel with the third diode D23. The second diode D22 becomes the input of the output voltage of ch1, and the third diode D23 becomes the input of the output voltage of ch2.

One end of the capacitor C11 is connected between the diode D23 and the resistor R25, and the other end is grounded. The Zener diode ZD12 is connected between the low voltage VccL and the emitter of the driver transistor Q25.
Is connected. According to the above-described circuit, similarly to the circuit of the first embodiment, after the output voltage Vout exceeds the reference voltage Vref and the power supply voltage Vcc becomes the high voltage VccH, the output voltage Vout falls again below the reference voltage Vref. This is similar in that the output voltage Vout sometimes does not immediately switch to the low voltage VccL but maintains the high voltage VccH for a certain period of time.
The third embodiment has the same effect as the first embodiment, but has a slightly different circuit configuration and operation.

In the above-described power supply circuit, the output voltage Vout of the amplifier 21 is constantly applied to the second and third diodes D22 and D23.
Is entered via If the output voltage Vout is lower than the predetermined reference voltage Vref, the base potential of the driver transistor Q25 does not rise sufficiently and does not rise to a voltage sufficient to turn on VBE of the driver transistor Q25. Is OFF.

Then, since the collector current of the driver transistor Q25 does not flow, the power transistor M11 is also turned off, and the transistor Q2 at the final output stage of the amplifier 11 is turned off.
1, the collector of Q22 is supplied with the low voltage VccL as the power supply voltage Vcc via the first diode D21.
When the output voltage Vout is higher than the predetermined reference voltage Vref, the fifth resistor R25 and the second diode D
22, the base potential of the driver transistor Q25 input through the driver transistor Q25 rises,
BE becomes high voltage, driver transistor Q25 is ON
I do.

Then, since the collector current of the driver transistor Q25 flows, the power transistor M21 is also turned on, and the transistor Q2 at the final output stage of the amplifier 21 is turned on.
1, Q22 have a power transistor M21
Is supplied with a high voltage VccH. The operation of the above circuit when the output voltage Vout falls below the reference voltage Vref again after the output voltage Vout exceeds the reference voltage Vref and the power supply voltage Vcc becomes the high voltage VccH, and will be described below.

In this case, for example, in the case of ch1, even if the output voltage Vout inputted from the second diode D22 decreases, the capacitor C11 is charged when the voltage becomes high VccH. This capacitor C
The base potential of the driver transistor Q15 does not immediately decrease until the charge charged in the transistor 11 is discharged. The time required for the base potential to sufficiently decrease falls in a time determined by the time constant of the fifth resistor R25 and the capacitor C11.

For this reason, the emitter potential of driver transistor Q25 does not immediately decrease, and driver transistor Q25 maintains the ON state for a while, whereby power transistor M21 maintains the ON state for a while. . Therefore, the power supply voltage Vcc does not immediately switch from the high voltage VccH to the low voltage VccL, but maintains the state of the high voltage VccH for a certain period of time.

As a result, as shown in FIG. 2, even when the high-frequency output voltage Vout is output, the power supply voltage Vcc does not immediately switch from the high voltage VccH to the low voltage VccL. Neither M21 nor the driver transistor Q25 repeats the ON / OFF operation at high speed, and the power supply voltage Vcc is changed to the high voltage VccH.
It will be maintained as it is.

Therefore, no matter how the high-frequency output voltage Vout is output, the situation where the change in the power supply voltage Vcc cannot follow the change in the high-frequency output voltage Vout does not occur. It is possible to achieve the same effect as in the first embodiment in that the distortion can be suppressed.

[0041]

As described above, according to the power supply circuit according to the present invention, when the output voltage rises and the power supply voltage reaches the second power supply voltage and then the output voltage falls again, the driver transistor The emitter potential of the driver transistor does not immediately decrease, and the driver transistor is turned on for a while.
N state so that the power transistor
The N state will be maintained for a while.

For this reason, the power supply voltage does not immediately switch from the second power supply voltage to the first power supply voltage, but changes the state of the second power supply voltage for a certain period of time determined by the time constant of the capacitor and the fifth resistor. keeping. As a result, even when a high-frequency output voltage is output, the power supply voltage is not immediately switched from the second power supply voltage to the first power supply voltage but is in a hold state. The ON / OFF operation is not repeated at a high frequency.

Therefore, no matter how the high-frequency output voltage is output, the change in the power supply voltage cannot follow the change in the high-frequency output voltage, so that the conventional distortion can be suppressed, and the hysteresis operation is performed. As compared with the conventional circuit, the distortion factor can be improved, and the sound quality of the amplifier can be improved.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a power supply circuit according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating an operation of the power supply circuit according to the embodiment of the present invention.

FIG. 3 is a circuit diagram of a power supply circuit according to a second embodiment of the present invention.

FIG. 4 is a circuit diagram of a conventional power supply circuit.

FIG. 5 is a diagram illustrating an operation of a general switching type power supply circuit.

FIG. 6 is a diagram illustrating a conventional problem.

[Explanation of symbols]

 M11, M12 Power transistor Q15, Q25 Driver transistor Q11-Q14 Transistor at last stage of amplifier Q16 Buffer transistor R11, R21 First resistor R12, R22 Second resistor R13, R23 Third resistor R14, R24 Fourth resistor R15 , R25 Fifth resistor D11, D21 First diode D12, D22 Second diode D13, D23 Third diode ZD11, ZD12 Zener diode C1, C11, C12 Capacitor Vout Output voltage Vcc Power supply voltage VccH High voltage (second Power supply voltage) VccL Low voltage (first power supply voltage)

Claims (2)

[Claims] A power supply voltage is supplied to an amplifier circuit for amplifying and outputting an input signal, and one of a first power supply voltage and a second power supply voltage higher than the first power supply voltage is supplied to the amplifier circuit. A power supply circuit which is selected according to a change in the output voltage of the amplifier circuit and is supplied to the amplifier circuit, comprising: a power transistor comprising a MOSFET;
Resistance, Zener diode, driver transistor,
A buffer transistor, a first diode, a second diode serving as an input of the output voltage, a third diode serving as an input of the output voltage, and a capacitor, wherein the source of the power transistor is the second power supply And the gate of the power transistor is connected to the first
And the drain of the power transistor is connected to the amplifier circuit, and the power supply transistor is connected to the first power supply voltage via the first diode. A gate connected to the collector of the driver transistor via a second resistor, and a base and an emitter of the driver transistor connected to a third
And the emitter of the driver transistor is connected to the emitter of the buffer transistor. The base of the driver transistor has a fifth resistor,
A third diode is connected in series, the second diode is connected in parallel with the third diode, one terminal of the capacitor is connected to the emitter of the driver transistor, and the other terminal is connected to the fifth terminal. The collector of the buffer transistor is grounded, the base is grounded via a fourth resistor, and is connected between the first power supply voltage and the base of the buffer transistor. A power supply circuit, wherein the Zener diode is connected to the power supply circuit. 2. A power supply voltage is supplied to an amplifier circuit for amplifying and outputting an input signal, and one of a first power supply voltage and a second power supply voltage higher than the first power supply voltage is supplied to the amplifier circuit. A power supply circuit which is selected according to a change in an output voltage of an amplifier circuit and supplied to the amplifier circuit, comprising: a power transistor, first to fifth resistors, a Zener diode, a driver transistor, a first diode, and the output voltage. A second diode serving as an input part of the power supply, a third diode serving as an input part of the output voltage, and first and second capacitors; a source of the power transistor is connected to the second power supply voltage; A gate connected to the second power supply voltage via a first resistor, the gate connected to a collector of a driver transistor via a second resistor, a drain of the power transistor Is connected to the amplifier circuit, and simultaneously connected to the first power supply voltage via a first diode. The base and the emitter of the driver transistor are connected by the third resistor, and the emitter is connected to the second resistor. A grounded fourth resistor is connected in parallel with the second capacitor via a capacitor, and a fifth resistor is connected to the base of the driver transistor.
A third diode is connected in series, the second diode is connected in parallel with the third diode, and one terminal of the first capacitor is connected between the third diode and the fifth resistor. A power supply circuit, wherein the zener diode is connected between the first power supply voltage and an emitter of the driver transistor.