JPH0119286B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a power amplifier including a class B push-pull output circuit.

As a protection circuit for the output transistor in a power amplifier including a class B push-pull output circuit,
ASO (safe operating area) limiters are well known.

This ASO limiter detects the operating state of the output transistor and turns on a limiter transistor provided between the input and output of the output transistor to limit the operation of the output transistor that deviates from the ASO. However, this limiter transistor forms an undesirable high-frequency positive feedback loop between the input and output of the output transistor, so it has the disadvantage that oscillation occurs when the ASO limiter operates.

Therefore, a protection circuit as shown in the block diagram of FIG. 2 was devised by the inventor of the present invention.

When the operating trajectory of the output transistor of the power amplifier Amp exceeds the preset ASO detection level,
The ASO detection circuit discharges the capacitor C and inverts the Schmitt circuit SM. Bias control circuits B and C that receive this inverted signal are power amplifiers.
This disables the bias circuit of the amplifier and turns off at least the output transistor.

By turning off this output transistor, the detection signal of the ASO detection circuit disappears, so after a certain period of time determined by the time constant of the capacitor C and the resistor R, the Schmitt circuit SM is inverted again, and the bias control circuits B and C control the power amplifier. Amp is automatically restarted.

Due to the time constant circuit of the capacitor C and the resistor R, the power amplifier Amp repeats intermittent operation when an accident such as a load short circuit continues.

Note that without the time constant circuit and Schmitt circuit, a closed loop would occur between the power amplifier Amp and the protection circuit, resulting in oscillation.

This protection circuit has the disadvantage that a capacitor is required for automatic recovery.

An object of the present invention is to provide a power amplifier with a simplified circuit.

According to the basic feature of the invention, a capacitor for automatic recovery in the protection circuit and a capacitor for eliminating power supply ripples in the bias circuit of the power amplifier are used together.

Hereinafter, this invention will be explained in detail together with examples.

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

In this example, although not particularly limited, the circuit elements configured in the partial IC surrounded by dotted lines are:
It is formed into one silicon chip by a well-known semiconductor manufacturing method. Further, the numbers surrounded by circles indicate terminal numbers.

The first stage transistor amplifier circuit is composed of the following circuit.

A pnp transistor Q ₁₀ whose base is connected to the No. 1 terminal to which the input signal V _IN is applied, the base of this transistor Q ₁₀ , the emitter, and a diode-type transistor (which may be a diode and the same applies hereinafter) Q ₁₁ . npn transistor with input voltage V _IN applied to the base through the emitter and base
_Q14 constitutes the first stage transistor amplifier circuit.

A pnp transistor _Q12 , which constitutes a constant current load via a transistor _Q11 , is provided at the emitter of the transistor _Q10 . A pnp transistor _Q13 , which constitutes a constant current load, is provided at the collector of the transistor _Q14 .

The emitter of transistor _Q14 constitutes a negative feedback input terminal and is connected to a current source circuit composed of transistor _Q15 and resistor _R18 via resistor _R19 for voltage gain setting. A capacitor C ₁₀₂ for AC grounding is provided at the other end of this resistor R ₁₉ via the No. 7 terminal.

The current amplification signal of the first stage amplifier circuit is a transistor
It is obtained from the collector of Q ₁₄ and transmitted to the base of the amplification transistor Q ₁₆ that constitutes the class A voltage amplification circuit.

The class A voltage amplification circuit consists of a Darlington type transistor Q ₁₆ with the transistor Q ₁₆ on the input side,
Q ₁₇ and a transistor Q ₈ as a constant current load provided on the collector side of the output transistor Q ₁₇
It is composed of

A diode-type transistor _Q22 is provided between the collectors of the transistor _Q17 and the transistor _Q8 , which constitutes a part of an idling bias circuit of a class B push-pull output circuit to be described later.

The collector of the input transistor Q ₁₆ is connected to the collector of the transistor Q ₁₇ via this transistor Q ₂₂ .

Furthermore, capacitors C ₁ and C ₂ connected in series for phase compensation are provided between the base of the transistor Q ₁₆ and the collector of the transistor Q ₁₇ .

A resistor R ₂₁ forming a negative feedback circuit of the class B push-pull output circuit is provided between the connection point of the capacitors C ₁ and C ₂ and the output terminal of the class B push-pull output circuit.

The collector of transistor Q ₁₇ is transmitted to the negative half-wave output circuit via anti-oscillation resistor R ₂₃ .

The negative half-wave output circuit consists of an output transistor _Q21 driven by an inverted signal formed by a pnp transistor _Q20 .

On the other hand, the transistor through transistor Q ₂₂
The output voltage of Q ₁₇ is passed through a similar resistor R ₂₄ to the positive half-wave output circuit. The positive half-wave output circuit consists of an npn transistor _Q18 in Darlington form,
The input _{transistor Q18} drives the output transistor _Q19 .

Both output transistors Q ₁₉ and Q ₂₁ are connected in cascade to perform a class B push-pull operation. As an idling bias circuit for these output circuits, the following circuit is provided between the emitter of transistor Q ₂₀ and the connection point of transistors Q ₁₉ and Q ₂₁ as output terminals.

The emitter of transistor Q ₂₀ is connected to the emitter of transistor Q ₂₃ via a resistor R ₂₅ .

A bootstrap power supply voltage V _CC ' is applied to the collector of transistor _Q23 . The output voltage is applied to the base via a constant voltage circuit formed by the voltage between the base and emitter of the transistor Q ₂₆ and the diode-type series transistors Q ₂₄ and Q ₂₅ provided between the base and the collector. is being applied. A resistor _R26 is provided between the base and emitter of the transistor _Q26 to allow a constant bias current to flow through the transistors _Q24 and _Q25 .

Furthermore, a constant current transistor _Q9 that supplies a bias current is provided at the base of the transistor _Q23 .

The above bias circuit and transistors Q ₁₈ , Q ₁₉ ,
Q ₂₀ and transistor Q ₂₂ constitute a DC closed loop, and the idling current of output transistors Q ₁₉ and Q ₂₁ is set.

On the other hand, a constant voltage is formed by the Zener diode ZD and the resistor _R4 .

This constant voltage is applied to the emitter follower transistor
Via _Q2 , it is used as a power supply voltage for the Schmitt circuit and bias control circuit that constitute the protection circuit described later.

Further, a constant voltage divided by resistors R ₅ to _{R 7} provided at the emitter of transistor Q ₂ is used as a power supply voltage for the first stage amplifier circuit.

That is, a voltage via resistor R ₅ is applied to the emitters of transistors Q ₁₂ and Q ₁₃ that constitute a constant current load. Also, between the resistors R ₅ and R ₆ ,
A diode-type transistor Q ₃ is provided and forms a constant current circuit together with transistors Q ₁₂ and Q ₁₃ .

In addition, as a bias circuit for the above power amplifier,
The following circuit is provided.

A transistor Q4 whose collector and base are connected via resistors _R9 and _R10 and _the base and emitter of a transistor _Q5 , and an emitter resistor _R11 are connected in series. This creates a current proportional to the power supply voltage V _CC . This current is
The current flows to transistor _Q15 , which together with _Q4 forms a current mirror circuit. By passing this current through resistor R ₂₀ , the output DC voltage is reduced to the midpoint voltage (V _CC /
2). Note that this resistance R ₂₀ is the resistance
Together with _R19 , it forms a feedback circuit for voltage gain setting.

On the other hand, a resistor _R13 is provided at the base of the transistor _Q4 to form a constant current. Therefore, the constant voltage formed by the resistors R ₁₃ and R ₁₂ through which a constant current flows, is applied to the base of the transistor Q ₆ to drive the transistor Q ₇ which forms a current mirror circuit together with the constant current transistors Q ₈ and Q ₉ . do.

Note that the capacitor C ₁₀₁ for removing the power supply ripple component of these bias current circuits is connected to a resistor.
It is connected to the connection point of R ₉ and R ₁₀ via the No. 8 terminal.

Output transistor forming a positive half-wave output signal
The collector of Q ₁₉ is provided with a resistor R ₂₉ that converts the collector current into a voltage signal. Furthermore, a diode-type transistor Q ₂₇ and a resistor R ₃₀ are provided between the collector and emitter of the transistor Q ₁₉ .
The voltage between the collector and emitter of transistor _Q19 is replaced by the voltage between the base and emitter of transistor _Q27 .

A voltage indicating the operating state of the output transistor _{Q19 formed by the resistor R29 and the transistor Q27 is applied} between the base and emitter of the transistor _Q28 . The collector of the transistor Q ₂₈ is provided with a transistor Q ₃₇ and a resistor R ₃₈ that constitute a constant current circuit through which a bias current flows.

At the base of transistor Q ₃₇ there is a transistor
A constant voltage similar to that formed by Q ₃₆ is applied.

Differential transistors Q ₃₁ and Q ₃₂ and an input transistor Q ₃₀ for level shifting constitute a Schmitt circuit. The voltage formed by the ripple removal capacitor _C101 is applied to the base of the input transistor _Q30 , and the level is set so that the transistors _Q30 and _Q31 are turned on.

A current mirror circuit as a bias control circuit is provided at the collector of the transistor Q ₃₂ from which the inverted output of the Schmitt circuit is obtained, which is composed of a transistor Q ₃₃ having a multi-collector configuration and a transistor Q ₃₄ . A resistor R ₃₂ is provided at the emitter of the transistor Q ₃₄ , and a small current is used as a charging current for the capacitor C ₁₀₁ for automatic restart.

In addition, the inverted output signal from the output side collector of transistor Q ₃₃ is transmitted to transistor Q ₃₈ which turns off constant current transistor Q ₆ and output transistor
It is transmitted to the base of transistor _Q39 which turns off _Q19 . In addition, the other inverted output signal is connected to a transistor to prevent pop noises during automatic restart.
Connected to the emitter of Q ₁₄ and charges capacitor C ₁₀₂ .

Note that the pnp transistor _Q1 , to which the Zener constant voltage is applied to the base and the power supply voltage _VCC divided by the resistors _R1 and _R2 is applied to the emitter, constitutes a surge voltage detection circuit. Then, the collector voltage is applied to the transistors Q ₃₈ and Q ₃₉ . Further, a transistor _Q40 , to which a constant voltage formed by a resistor _R7 is applied to its base, constitutes a thermal shutdown circuit.

The ASO protection operation of this embodiment circuit will be explained next.

Based on the voltage between the base and emitter due to the bias constant current of transistor _Q28 ,
If the output current and/or emitter/collector voltage of _Q19 exceeds its tolerance range, the transistor
Q ₂₉ turns on and discharges capacitor C ₁₀₁ . When the capacitor C ₁₀₁ is not discharged below a predetermined voltage, which will be described later, the transistor Q ₃₁ in the Schmitt circuit is on and the transistor Q ₃₂ is off.

Now, assuming that the constant voltage at the emitter of the transistor _Q33 is _VZ , the emitter voltage _V1 of the transistor _Q31 can be approximately expressed by the following equation 1).

V ₁ ≒ V _Z・R ₃₅ /R ₃₅ +R ₃₆ ...1) In the above equation 1), from the collector-emitter voltage of transistor Q ₃₁ and resistor R ₃₄ to resistor R ₃₅
Since the current flowing into the circuit is minute, it is ignored.

Also, if the potential of capacitor C ₁₀₁ is V _C , then
A voltage of V _C −V _BEQ30 is taken out at the emitter of transistor _Q30 . In addition, V _BEQ30 is a transistor
This is the base-emitter threshold voltage of _Q30 .

Therefore, if the current flowing through the series resistance of resistor R ₃₃ and resistor R ₃₄ is I ₁ , current I ₁ can be expressed by the following equation 2).

I ₁ ≈V _C −V ₁ −V _BEQ30 /R ₃₃ +R ₃₄ ...2) Now, the condition for turning off the transistor Q ₃₁ occurs across the resistor R ₃₄ as shown in equation 3) below. The voltage V _R34 , that is, I ₁ · R ₃₄ is the base-emitter threshold voltage V _BEQ31 of the transistor Q ₃₁
When it is smaller than

V _R34 <V _BEQ31 …3) Capacitor that can satisfy the above formula 3)
The C ₁₀₁ potential is expressed by the following equation 4) from equations 1), 2), and 3) above.

V _C <V ₁ +V _BEQ31 (1+R ₃₃ /R ₃₄ )+V _BEQ30 ...4) When the capacitor C ₁₀₁ is discharged until it satisfies the above equation 4), the transistor Q ₃₁ is turned off. This inverts the Schmitts circuit and turns it into a transistor.
Since Q ₃₂ is turned on, transistor Q ₃₃ is turned on. Then, in order to turn on transistor _Q38 and turn off constant current transistor _Q6 , constant current transistors _Q8 and _Q9 are turned off, and all transistors _Q18 to _Q21 forming the output circuit are turned off. In particular, output transistor Q ₁₉ is replaced by transistor Q ₃₉
By turning on the switch, the switch can be turned off reliably even if there is some leakage current. This performs ASO protection operation.

Due to this ASO protection operation, an ASO detection signal cannot be obtained from transistor _Q28 , so transistor _Q29 is turned off. Now transistor Q ₃₄
Since the transistor Q ₃₂ is on, it is in the on state and the capacitor C ₁₀₁ can be charged.

Assuming that the emitter potential of transistor Q ₃₂ when transistor Q ₃₂ is on is V ₁ ', V ₁ ' can be expressed by the following equation 5).

V ₁ ′≒(V _Z −V _BEQ33 )・R ₃₅ /R ₃₅ +R ₃₇ …5) In the above equation 5), V _BEQ33 is the transistor
It shows the base-emitter threshold voltage of _Q33 . In addition, in the above equation 5), the transistor Q ₃₂
The collector-emitter voltage and the current flowing from resistor _R34 to resistor _R35 are so small that they are ignored.

Further, the current I ₁ ' flowing through the resistor R ₃₄ can be expressed by the following equation 6).

I ₁ ′≒V _C −V ₁ ′−V _BEQ30 /R ₃₃ +R ₃₄ ...6) Now, the condition for turning on transistor Q ₃₁ is the voltage V' generated across resistor R ₃₄ as shown in equation 7) below. This is when _R34 , that is, I ₁ '·R ₃₄ exceeds the base-emitter threshold voltage V _BEQ31 of transistor Q ₃₁ .

V′ _R34 ≧V _BEQ31 …7) A capacitor that can satisfy the above formula 7)
The voltage V′ _C of C ₁₀₁ can be expressed by the following equation 8).

V′ _C ≧V ₁ ′+V _BEQ31 (1+R ₃₃ /R ₃₄ )+V _BEQ30 …8) When capacitor C ₁₀₁ is charged until it satisfies the above equation 8), transistor Q ₃₁ is turned on again and transistor Q ₃₂ is turned on. It turns off and automatically restarts.

Note that the resistor _R36 in the Schmitt circuit is set larger than the resistor ₃₇ . This is to provide hysteresis to the Schmitt circuit.

Until this automatic restart, the collector current of the transistor Q ₂₃ , which is the inverted output signal of the Schmitt circuit, charges the external capacitor C ₁₀₂ via the resistor R ₁₉ , which may cause a pop sound after the automatic restart. do not have.

In this embodiment, the capacitor for automatic recovery in the ASO protection circuit and the capacitor _C101 for removing power supply ripples can be used together, so the circuit can be simplified. In particular, when fabricating a monolithic IC, the number of external terminals and external capacitors can be reduced, making it possible to reduce the cost of the power amplifier.

The present invention is not limited to the embodiments described above, and the specific circuit configuration of the power amplifier and the specific configuration of various protection circuits including the ASO protection circuit may be any type as long as they perform the same operations as described above. Good too.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention;
Figure 2 shows the ASO system devised prior to this invention.
FIG. 2 is a block diagram showing an example of a protection circuit.

Claims (1)

[Claims] 1. A class B push-pull output circuit having an output transistor _Q19 connected between the power supply voltage terminal _Vcc and the output terminal _Vput , and a power supply ripple elimination capacitor forming part of the time constant circuit. C ₁₀₁ and includes at least a bias circuit that supplies a bias to the output transistor Q ₁₉ , a detection circuit that detects the operating state of the output transistor Q ₁₉ , and a bias control circuit that controls the bias circuit. The bias control circuit further includes discharging means for discharging the charge of the capacitor C ₁₀₁ in response to the output of the detection circuit, and a discharging means for discharging the charge of the capacitor C 101 when the potential of the capacitor C ₁₀₁ is equal to or lower than a first potential. a first detection means for detecting whether or not the capacitor C 101 is present; a charging means for charging _{the capacitor C 101 in} response to the first detection means; a second detection means for detecting whether or not the transistor Q19 exists, and the first detection means controls the bias circuit so as to cut off the bias supply to the transistor _Q19 ; The second sensing means detects the transistor _Q19 .
A power amplifier, characterized in that the power amplifier controls the bias circuit so that bias supply to the power amplifier is restored. 2. In the power amplifier according to claim 1, the detection circuit includes the output transistor Q ₁₉
A power amplifier characterized in that it is an ASO detection circuit that detects a collector current and a collector-emitter voltage.