JPH0119285B2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to a low frequency power amplifier circuit.

Prior to the present invention, a circuit as shown in FIG. 1 was proposed as an ASO (safe operating area) protection circuit for an output transistor in a low frequency power amplifier circuit. This circuit consists of a first-stage amplifier circuit 1 and an amplifying transistor Q ₁₉ to which the amplified output is applied to the base.
and a constant current load transistor _Q16 provided at the collector of the A-class voltage amplification circuit.
In a low frequency power amplifier circuit including a quasi-complementary class B push-pull output circuit to which this voltage amplified output is applied to the input, the ASO of the output transistor _Q28 on the power supply voltage side constituting the class B push-pull output circuit is detected. By forcibly turning off the constant current transistor Q ₁₆ and cutting off the bias current, a protection operation is performed in which the push-pull transistors Q ₂₇ and Q ₂₈ on the power supply voltage side are turned off. That is, the above constant current transistor Q ₁₆
constitutes a current mirror circuit together with transistor _Q15 in diode form, and the constant current transistor
This is to pass a constant current formed by _Q8 . And at the base of this constant current transistor _Q8 ,
The base of transistor Q ₇ in the constant voltage generation circuit composed of resistor R ₁₃ and transistors Q ₆ and Q ₇ ,
A constant voltage is applied between the emitters, and the constant current is cut off by turning on transistor _Q2 , which forcibly turns off these transistors _Q7 and _Q8 . This transistor Q ₂ is controlled by a switch control circuit 3 to which the output of the ASO detection circuit 2 is applied.
The voltage drop across the resistor R ₃₅ that converts the collector current of the output transistor Q ₂₈ into a voltage signal and the transistor
The voltage between the collector and emitter of Q ₂₈ is input,
ASO detection operation is performed.

Note that the diodes (including diode-type transistors, the same applies hereinafter) Q ₂₃ to _{Q 25} serve to reduce crossover distortion of the Darlington-type drive transistors Q ₂₇ , Q ₂₈ and the inverted Darlington-type drive transistors Q ₂₁ , Q ₂₆ . It forms the bias voltage for the output terminal
The output signal of OUT is negatively fed back to the stage amplifier circuit 1 according to the resistance ratio of the resistors R ₂₂ and R ₂₀ to set the voltage gain. Capacitor C ₁₀₁ is an AC grounding capacitor, and 100% of the DC voltage is fed back.

The research conducted by the inventor of this application has revealed that the above-mentioned ASO protection circuit has the following problems during the protection operation.

FIG. 5 shows a cross-sectional view of a case where the constant current transistor _Q15 constituting the current mirror circuit in the circuit shown in FIG. 1 is formed in a monolithic semiconductor integrated circuit. In FIG. 5, P-sub is a semiconductor substrate whose conductivity type is P type, and the substrate is connected to a reference potential. Reference numerals 11 and 11' denote island regions of N type conductivity that are formed spaced apart from each other in the semiconductor substrate, and the island region 11 is the base region of the transistor _Q15 . In the island region 11, an emitter region 1 whose conductivity type is P type is provided.
2 and the collector region 13 are formed spaced apart from each other. That is, the above constant current transistor Q ₁₅
is composed of a lateral structure.

Emitter electrode E of the above constant current transistor _Q15
is connected to the power supply voltage Vcc, the base electrode B and the collector electrode C are connected in common, and the transistor Q ₁₆
connected to the base of.

As shown by the dashed line, the constant current transistor _Q15 includes a parasitic transistor Qp having the emitter region 12 of the constant current transistor _Q15 as an emitter, the island region 11 as a base, and the semiconductor substrate P-sub as a collector. exists. Note that a resistor Rp exists between the point X where minority carriers are injected into the base of the parasitic transistor Qp and the base electrode of the constant current transistor _Q15 .

Now, during the ASO protection operation by turning on the transistor _Q2 , the island region 11 and the semiconductor substrate P-
Semiconductor substrate P-sub via PN junction between sub
When minority carriers (holes) are injected into the island region 11 from From the emitter region 11 to the semiconductor substrate P-
A current of hfeQp・ip flows through sub. Note that hfeQp indicates the current amplification factor of the parasitic transistor Qp.

As the current hfeQp flows, a minute voltage ΔV _BE Qp as shown in the following equation 1) is generated at the base of the parasitic transistor Qp, that is, at the point X.

ΔV _BE Qp=K _T /qlnIeQp/IsQp...1) In the above equation 1), K is Boltzmann's constant, T
is temperature, q is charge, IsQp is parasitic transistor Qp
The reverse saturation current of , IeQp is the parasitic transistor Qp
shows the emitter current of

Since no bias is supplied to the base and collector electrodes of the constant current transistor _Q15 during the ASO protection operation, the voltage ΔV _BE Qp generated at the base of the parasitic transistor Qp is applied via the resistor Rp.

As a result, a base current as shown in the following equation 2) flows through the constant current transistor _Q15 .

I _B Q ₁₅ = IsQ ₁₅ exp (q/K _T ΔV _BE Qp) / hfeQ ₁₅ ...2) In the above equation 2), IsQ ₁₅ is the reverse saturation current of the constant current transistor Q ₁₅ , and hfeQ ₁₅ is the constant current transistor Q Shows a current amplification factor of ₁₅ .

Since the base of the transistor _Q16 is connected to the base electrode of the constant current transistor _Q15 , the same current as the base current of the constant current transistor _Q15 flows.

Note that the emitter current of the parasitic transistor Qp that flows when the parasitic transistor Qp operates is the current amplification factor of the parasitic transistor Qp.
Since hfeQp is extremely small (actually close to 1)
The voltage ΔV _BE Qp taken out to its base is also very small.

However, since the current amplification factors of the constant current transistor Q ₁₅ and transistor Q ₁₆ are extremely large compared to that of the parasitic transistor Qp (actually over 100), the current (leakage current) flowing through their respective collectors is large. becomes. The following equation 3) shows the current flowing to the collector of the transistor _Q16 .

icQ ₁₆ = I _BQ16 × hfe _Q16 = IsQ ₁₆ exp (q/K _T ΔV _BE Qp) ...3) The leakage current flowing to the collector of the above transistor Q ₁₆ further passes through the next stage transistors Q ₂₇ and Q ₂₈ to a high current amplification factor. A relatively large current flows through the collector of the output transistor _Q28 . This makes it impossible to perform a sufficient protective operation.

In particular, low-frequency power amplifier circuits (so-called power ICs) composed of monolithic ICs (semiconductor integrated circuits)
In this case, since elements are separated using a pn junction, it is extremely difficult to suppress the leakage current to zero, and the variation in the leakage current is also large, which causes a deterioration in product yield.

An object of the present invention is to provide a low frequency power amplifier circuit equipped with a protection circuit that can reliably prevent damage to an output transistor.

The basic configuration of the present invention is that an emitter electrode is connected to a power supply voltage Vcc, a collector electrode and a base electrode are connected in common, and are formed in a semiconductor substrate having a first conductivity type, and a a first semiconductor region having a conductivity type as a base region;
a second semiconductor region formed in the first semiconductor region spaced apart from each other and having the same conductivity type as the semiconductor substrate;
A constant current transistor _Q15 having a semiconductor region and a third semiconductor region as an emitter region and a collector region, respectively, the second semiconductor region as an emitter, the first semiconductor region as a base, and the semiconductor region as a base. A class A voltage amplification circuit whose load is a parasitic transistor Qp whose collector is a substrate, a constant current transistor _Q15 in which a resistor Rp exists between the base electrode of the constant current transistor and the base of the parasitic transistor Qp; A class B push-pull output circuit to which the amplified output signal is applied to the input, the output transistor ASO detection signal and the surge voltage detection signal, the constant current transistor Q ₁₅
In a low frequency power amplification circuit including a protection circuit that cuts off constant input current to the base and collector of the transistor, a resistor _R48 is connected between the base electrode and emitter electrode of the constant current transistor _Q15 , and the input constant current is cut off. The voltage drop V _R48 that occurs across the resistor R ₄₈ during cutoff is V _R48 = ΔV _BE Qp/Rp + R ₄₈ × R ₄₈ (where ΔV _BE Qp indicates the voltage between the base and emitter of the parasitic transistor). It is characterized in that it is determined.

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

FIG. 2 is a circuit diagram showing a basic embodiment of the present invention.

This circuit includes a first stage amplifier circuit 1 similar to that described above, an amplification transistor _Q19 to which the amplified output is applied to the base, and diodes _Q23 to _Q25 forming a bias circuit at the collector thereof. This circuit is used as a protection circuit in a low frequency power amplifier circuit including a class A voltage amplifier circuit configured with a constant current load transistor _Q16 and a quasi-complementary class B push-pull output circuit to which the voltage amplified output is applied to the input. In the example, B
By detecting the ASO of the output transistor _Q28 on the power supply voltage side that constitutes the class push-pull output circuit and forcibly turning off the constant current transistor _Q16 to cut off the bias current, the push-pull output circuit on the power supply voltage side is In addition to turning off transistors Q ₂₇ and Q ₂₈ , a switching transistor Q ₃₂ controlled by the ASO detection signal is connected between the base of the output transistor Q ₂₈ and the reference potential (OV) terminal.
is provided.

That is, the protection circuit by interrupting the bias current is similar to the above, and the protection circuit by blocking the bias current
_Q16 constitutes a current mirror circuit together with a diode-type transistor _Q15 , and allows a constant current formed by a constant current transistor _Q8 to flow therethrough. And at the base of this constant current transistor _Q8 ,
The base of transistor Q ₇ in a constant voltage generation circuit composed of resistor R ₁₃ and transistors Q ₆ and Q ₇ ,
An emitter-to-emitter voltage is applied, and by turning on the transistor Q ₂ provided between the base and emitter of these transistors Q ₇ and Q ₈ , the transistors Q ₇ and Q ₈ are forcibly turned off.
The above-mentioned constant current interrupting operation is performed.

The transistor _Q2 is controlled by a switch control circuit 3 to which the output of the ASO detection circuit 2 is applied. The ASO detection circuit 2 has the voltage drop across the resistor R ₃₅ that converts the collector current of the output transistor Q ₂₈ into a voltage signal and the voltage between the collector and emitter of the transistor Q ₂₈ applied to its input.
ASO detection operation is performed.

The switching transistor _Q32 and the transistor _Q2 are controlled in common by an ASO detection signal passed through a switch control circuit.

In this embodiment, even if there is a leakage current during the same protective operation as described above by turning on the transistor _Q2 , the amplified leakage current flowing through the transistor _Q27 is absorbed by turning on the transistor _Q32 , which operates in the same way, and is output. Since transistor _Q28 is completely turned off, reliable ASO protection operation is achieved. Therefore, if the sink current capability of transistor Q ₃₂ is set sufficiently large, even if transistor Q ₂ is omitted,
Similar to the above, reliable protection operation can be realized.

In addition, when a protection circuit is configured with two transistors Q ₂ and Q ₃₂ as in this embodiment, there is an advantage that a small-sized transistor can be used and the current consumption during protection operation can be reduced. .

In addition, in this embodiment, the ASO protection operation turns off the transistors Q ₁₅ , Q ₁₆ and the transistor Q ₂₇ , and conversely turns off the output transistor Q ₂₈ in a biased state, so that the collector of the transistor Q ₂₈ , Focusing on the fact that the emitter-to-emitter breakdown voltage is higher than that with the base open (no transistor Q ₃₂ ), by controlling the transistors Q ₂ and Q ₃₂ with the power surge voltage detection circuit 4, It also has the advantage that it can be used in common with a power surge protection circuit.

In addition, in this embodiment, in order to set the voltage gain in the low frequency power amplifier circuit as described above, resistors R ₂₀ and R ₂₂ and a capacitor O ₁₀₁ are connected between the output terminal OUT and the inverting input terminal of the first stage amplifier circuit 1. A feedback circuit is provided.

FIG. 3 is a circuit diagram showing another basic embodiment of the present invention.

In this embodiment, in a low frequency power amplifier circuit having an ASO protection circuit using _{the transistor Q 2 explained} in the embodiment of FIG. In order to prevent leakage current flowing through transistors Q ₁₅ and Q ₁₆ , a high resistance R ₄₈ is provided between the commonly connected bases and emitters of transistors Q ₁₅ and Q 16.

FIG. 6 shows a cross-sectional view of a case where the current transistor _Q15 constituting the current mirror circuit in the circuit shown in FIG. 3 is formed in a monolithic semiconductor integrated circuit.

In FIG. 6, the only difference from the drawing in FIG. 5 is that a resistor R ₄₈ is connected between the base electrode (collector electrode) of the constant current transistor Q ₁₅ and the power supply voltage Vcc. Descriptions that overlap with the above will be omitted.

In FIG. 6, when the parasitic transistor Qp operates now, a voltage ΔV _BE Qp as shown in the above equation 1) is generated at its base, that is, at the point X.
Since the resistor _R48 is connected between the base electrode (collector electrode) of the parasitic transistor _Q15 and the power supply voltage Vcc, the above ΔV is applied across the series resistance formed by the resistor _R48 and the resistor Rp. A potential of _BE Qp will be generated. Note that ix is the current flowing through this series resistor.

Therefore, the potential difference generated between the base electrode and the emitter electrode of the constant current transistor _Q15 can be expressed by the following equation 4).

ΔV _BE Q ₁₅ = ix.R ₄₈ = ΔV _BE Qp/R ₄₈ +Rp×R ₄₈ …4) In the above equation 4), if R ₄₈ ≪Rp,
Needless to say, ΔV _BE Q ₁₅ can be kept to a sufficiently smaller value than ΔV _BE Qp.

Since the voltage generated at the base electrode of the constant current transistor _Q15 is also applied to the base of the transistor _Q16 , the potential difference generated between the base-emitter voltage of the transistor _Q16 also has the same potential difference as shown in equation 4) above. It turns out.

Therefore, the leakage current flowing to the collector of constant current transistor Q ₁₅ and the collector of transistor Q ₁₆ is also reduced by R ₄₈ /R ₄₈ +Rp.

It is desirable that the value of the resistor R ₄₈ is sufficiently small compared to Rp, but it must have a somewhat high resistance in order to receive the input constant current of the transistor Q ₈ and to quickly turn on the constant current transistor Q ₁₅ . Since this is also necessary, the value is set based on the balance between the two.

For this reason, the leakage current generated in the collector of the transistor Q ₁₆ is kept extremely small, and the leakage current amplified by the transistors Q ₂₇ and Q ₂₈ in the next stage is also small. Therefore, situations that interfere with the ASO protection operation are eliminated. In this embodiment, since the resistor R ₄₈ is simply added, a reliable ASO protection operation can be realized extremely easily.

Incidentally, in order to achieve a more reliable protection operation, the transistor _Q32 according to the embodiment of FIG. 2 may also be added. In this case, since the leakage current of the transistor _Q16 is reduced by inserting the resistor ₄₈ against a relatively large leakage current, it is possible to reduce the size of the transistor _Q32 .

FIG. 4 shows a specific example circuit of the present invention, and the portion surrounded by dotted lines in the figure
The circuit elements constituting the IC are formed on a single-chip silicon substrate by a well-known semiconductor manufacturing method, and the numbers in circles indicate terminal numbers.

The first stage amplifier circuit 1 converts an input voltage signal into a current signal, and has a constant current transistor Q ₁₂ at its emitter and a diode-type transistor Q ₁₁ for level shifting in series, and a transistor Q 12 at its base.
Transistor to which the input signal from the terminal is applied
Q ₁₀ , an amplification transistor Q ₁₄ to which the emitter output voltage of transistor Q ₁₀ is applied to the base via a level shift transistor Q 11, and a constant current load transistor _{Q 13 provided} at the collector of this transistor Q ₁₄ . be done.

Note that the emitter of the transistor _Q14 is connected to the resistor _R20 for the gain setting and the constant current transistor.
Q ₉ is provided, resistor R ₂₀ and transistor Q ₉
A capacitor C ₁₀₁ for AC grounding is provided at the connection point with the collector via the No. 4 terminal.

Further, the emitter of the transistor _Q14 serves as an inverting input terminal of the first stage amplifier circuit 1, and 100% of the output DC voltage is fed back through the resistor _R22 , and an AC signal divided by the resistors _R22 and _R20 is fed back. , the voltage gain as a low frequency power amplifier circuit is set.

Furthermore, the constant voltage formed by the Zener diode ZD and the resistor _R5 is connected to the base of the transistor _Q3 ,
It is used as a power supply voltage for the first stage amplifier circuit 1 via the emitter.

Also, using the constant voltage at the emitter of this transistor Q ₃ as a reference, resistor R ₆ and transistor Q ₄
A low constant voltage obtained by subtracting the voltage drop across the resistor _R7 from the base-to-emitter voltage is applied to the bases of the constant current transistors _Q12 and _Q13 .

In addition, the resistor R ₉ divided by the above resistors R ₆ to R ₉ , etc.
The transistor _Q5 , to which a constant voltage is applied to the base, constitutes a detection transistor for thermal shutdown.

The collector current output signal of amplification transistor _Q14 is applied to the base of amplification transistor _Q18 .

This transistor Q ₁₈ is connected in a Darlington configuration with the transistor Q ₁₉ to constitute an amplification transistor of a class A voltage amplification circuit. Between the input and output of this amplification transistor, in other words, the transistor
A phase compensation capacitor _C1 is provided between the base of _Q18 and the collector of transistor _Q19 . A constant current load transistor Q ₁₆ similar to that described above is connected to the collector of the transistor Q ₁₉ via a diode-type transistor Q ₂₀ constituting a bias circuit, and a transistor Q ₁₈
The collector of this load transistor _Q16 is connected to the collector.

The constant current load transistor _Q16 forms a current mirror circuit together with the diode-type transistor _Q15 , and the constant current formed by the constant current transistor _Q8 flows to the transistor _Q15 as an input current. At the base of this constant current transistor _Q8 ,
A resistor R ₁₁ connected in series with the same resistor R ₁₃ as described above,
Based on the constant voltage between the base and emitter of transistor Q ₇ in the constant voltage generation circuit composed of R ₁₂ and transistors Q ₇ and Q _6, the constant voltage formed by series resistors R ₁₅ and R ₁₆ is applied to resistor R ₁₇ . applied via.

The collector output voltage of the transistor Q ₁₉ of this class A voltage amplification circuit is applied to the base of the inverted Darlington type drive transistor Q ₂₁ , which forms a negative half-wave output, via a resistor R ₂₄ to prevent oscillation. The output transistor _Q26 is driven by its collector output.

On the other hand, the collector output voltage of the transistor Q ₁₉ via the level shifting transistor Q ₂₀ is applied to the base of the Darlington type drive transistor Q ₂₇ which forms a positive half-wave output via the oscillation prevention resistor R ₂₅ similar to the above. The output transistor _Q28 is driven by its emitter output.

The emitter of the transistor _Q21 is connected to one end of the bias circuit via a resistor _R26 , the base of the transistor _Q22 , the emitter, and a resistor _R27 . This bias circuit consists of a constant voltage circuit in which a resistor R ₄₀ is provided between the base and emitter, diodes (including diode-type transistors) Q ₂₄ and Q ₂₅ are provided between the base and collector, and the other end is Connected to the output terminal. The emitter of the output transistor Q ₂₈ and the collector of the output transistor Q ₂₆ are commonly connected to this output terminal, and an output signal is obtained from the No. 6 terminal.

Transistor Q ₂₃ that constitutes the above bias circuit
At the collector connection point of the above transistor Q ₁₅ ,
A bias current is supplied from a constant current transistor _Q17 , which together with _Q16 constitutes a current mirror circuit.

Further, the collector of transistor _Q22 is connected to the No. 7 terminal, which is the bootstrap power supply voltage.

This bootstrap power supply voltage is used as a power supply voltage for transistors Q ₁₅ to _{Q 17} and drive transistor Q ₂₇ that constitute a current mirror circuit.

In this embodiment, in order _to make the clipping waveform on the power supply voltage side a soft clipping waveform in the case _of an over-input signal, there is , a transistor _Q29 is provided.

At the base of this transistor Q ₂₉ there is a resistor R ₁₁
A predetermined voltage divided by ~ _R13 etc. is applied, and the emitter voltage of the drive transistor _Q27 exceeding this voltage turns it on, lowering the gain of the class A pressure amplifier circuit and soft clipping the output waveform. This prevents the generation of high frequencies.

In the ASO detection circuit 2, the voltages at both ends of a resistor R ₃₅ , which is provided at the collector of the output transistor Q ₂₈ that forms a positive half-wave output and converts the collector current into a voltage signal, are applied to the respective emitters, and the bases are connected in common. transistors Q ₃₀ and Q ₃₁ , a constant current transistor Q ₃₃ provided at the collector of one transistor Q ₃₁ , and the other transistor Q ₃₀
It consists of a commonly connected base and collector, and a resistor _R36 provided between the output terminal and the output terminal.

In this circuit, the collector current of transistor Q ₃₁ is the emitter voltage difference between transistors Q ₃₀ and Q ₃₁ ,
In other words, a current flows that is proportional to the product of the collector current of transistor Q ₂₈ and the voltage between the collector and emitter of transistor Q ₂₈ , and when it exceeds the ASO detection reference current formed by transistor Q ₃₃ , the switch control Activate circuit 3.

The switch control circuit 3 is a differential transistor
A Schmitt circuit using Q ₃₇ and Q ₃₈ is used.

In other words, the emitter voltage of the transistor _Q36 , to which the power supply voltage divided by the resistors _R11 to _R13 , etc. is applied to the base, is further divided by the resistors _R39 , _R41 , _R42, etc., and one of the Applied to the base of differential transistor _Q37 .

The base of the other differential transistor _Q38 is connected to the collector of one differential transistor _Q37 . In addition, the collector of this transistor _Q38 has a multi-collector structure transistor.
A current mirror circuit consisting of transistor Q ₃₉ and transistor Q ₃₅ is provided via resistor R ₄₄ , and the collector current of transistor Q ₃₅ is fed back to the base of transistor Q ₃₆ on the input side.

Therefore, the output transistor Q ₂₈ in the ASO
For operation, transistor Q ₃₄ , which is provided between the base of transistor Q ₃₆ and the reference voltage and controlled by the ASO detection current, turns off, transistors Q ₃₆ and Q ₃₇ turn on, and transistor Q ₃₈ The Schmitt circuit is stable in the off state.

When the ASO detection current flows to the base of transistor _Q34 , transistor _Q34 turns on and inverts the Schmitt circuit. Therefore, the transistor _Q38 turns on, turning on the transistor _Q39 , and its collector current turns on the transistor _Q2 , which turns off the constant current transistor _Q8 , as well as the output transistor. Transistor _Q32 , which reverse biases _Q28 , turns on and performs the ASO protection operation.

Note that the ASO protection operation can be canceled only if the transistor
From Q ₃₅ , capacitor C ₁₀₀ connected to terminal 1
Waiting for the charge up time is done. When power is input, the Schmitt circuit is inverted for the charge-up time of capacitor C ₁₀₀ , and one collector output of transistor Q ₃₉ sets the inverting input of first stage amplifier circuit 1 to high level, so that the bias system is in an unstable state. This prevents the output midpoint voltage from rising abnormally.

Note that the transistor Q ₄₁ , which is made into a diode form by the transistor Q ₄₀ , and the resistor R ₄₇ constitute a constant voltage generation circuit, and the constant voltage between the base and emitter of the transistor Q ₄₁ forms the ASO detection constant current. Q is applied to the base of ₃₅ .

The power surge voltage detection circuit 4 has a constant voltage formed by a Zener diode ZD applied to its base,
It consists of a transistor Q ₁ to which a voltage obtained by dividing the power supply voltage Vcc by resistors R ₁ and R ₂ is applied to the emitter, and the detection voltage at resistor R ₃ is common to the bases of the protection transistors Q ₂ and Q ₃₂ . is applied, and a surge protection operation is performed.

Further, the thermal shutdown detection transistor _Q5 is turned on due to a decrease in the threshold voltage between the base and emitter due to a rise in temperature, and turns off the constant current transistor _Q8 , thereby preventing thermal damage to the output transistor.

Furthermore, in this embodiment, a resistor R ₄₈ similar to that described above is provided to reduce leakage current of transistors Q ₁₅ and Q ₁₆ during ASO protection and surge protection operations. This resistor R ₄₈ is connected to the transistor Q ₁₅ ~
The transistor _Q15 for surge protection is used to short-circuit the base and emitter of _Q17 .
~ This contributes to improving the withstand voltage of _Q17 .

Note that the power supply voltage Vcc is applied to terminal 8,
A boot-trap capacitor _C102 is provided between the No. 7 terminal and the No. 6 output terminal. Capacitor C ₁₀₃ is the output capacitor,
Capacitor C ₁₀₄ is a capacitor for removing power supply ripples.

As mentioned above, in a low frequency power amplifier circuit configured with a monolithic IC like this embodiment circuit, leakage current is likely to occur during ASO or surge protection operation, so application of the present invention ensures reliable protection. operation can be realized.

The present invention is not limited to the above-mentioned embodiments, and the first stage amplifier circuit, class A voltage amplifier circuit, class B push-pull output circuit that constitute the low frequency power amplifier circuit, and their auxiliary circuits have the above-mentioned functions. Various circuit modifications can be made as long as the circuit has the following characteristics.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an example of a low frequency power amplifier circuit proposed prior to this invention, FIGS. 2 and 3 are circuit diagrams showing a basic embodiment of this invention, and FIG. The figure is a circuit diagram showing a specific embodiment of the present invention, and FIG. 5 is a cross-sectional view of the constant current transistor _Q15 in the circuit shown in FIG. 1 formed in a monolithic semiconductor integrated circuit. , 6th
The figure shows a cross-sectional view of the constant current transistor _Q15 in the circuit shown in FIG. 3 formed in a monolithic semiconductor integrated circuit. 1... First stage amplifier circuit, 2... ASO detection circuit, 3...
Switch control circuit, 4...Surge voltage detection circuit.

Claims (1)

[Claims] 1. The emitter electrode is connected to the power supply voltage Vcc, the collector electrode and the base electrode are connected in common, and the first
A first semiconductor region formed within a semiconductor substrate having a conductivity type and having a conductivity type opposite to that of the semiconductor substrate is used as a base region; A constant current transistor Q ₁₅ in which a second semiconductor region and a third semiconductor region having the same conductivity type as the substrate serve as an emitter region and a collector region, respectively, and the second semiconductor region serves as an emitter and the 1
A parasitic transistor Qp having the semiconductor region as a base and the semiconductor substrate as a collector, and a constant current transistor _Q15 having a resistor Rp between the base electrode of the constant current transistor and the base of the parasitic transistor Qp as a load. a class A voltage amplification circuit, a class B push-pull output circuit to which this amplified output signal is applied to its input, and an output transistor.
In a low frequency power amplifier circuit including a protection circuit in which constant current input to the base and collector of the constant current transistor Q ₁₅ is cut off by an ASO detection signal and/or a surge voltage detection signal, the base electrode of the constant current transistor Q ₁₅ and A resistor _R48 is connected between the emitter electrode and the voltage drop V _R48 that occurs across the resistor _R48 when the input constant current is cut off is V _R48 = ΔV _BE Qp/Rp + R ₄₈ × R ₄₈ (here, ΔV _BE Qp indicates the base-emitter voltage of a parasitic transistor. 2. The low frequency power amplifier circuit according to claim 1, wherein the transistor controlled by the ASO detection signal and the surge voltage detection signal is provided between the base of the output transistor and the reference potential terminal.