JPS6226204B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a power amplifier equipped with a single-ended push-pull amplifier circuit (hereinafter referred to as SEPP amplifier circuit).

As shown in FIG. 1, a conventional power amplifier includes output transistors 20 and 21 connected in SEPP, drive transistors 19 and 12 that drive the output transistors 20 and 21, respectively, and a pre-drive transistor that drives the drive transistors 19 and 12. 10, load resistors 15 and 16 of the front drive transistor 10, bias source transistor 11 and resistor 13 of the drive transistor 12, bias determination and temperature compensation transistor 14 of the output transistors 20 and 21, and diode 1.
The common connection point of the emitter of the output transistor 20 and the collector of the output transistor 21 is connected to the output terminal 4 and is further grounded through the output capacitor 7 and the load resistor 8. load resistance 1
5 and 16 are connected to the bootstrap terminal 3 and further connected to the output terminal 4 through the bootstrap capacitor 6.

During AC operation in the power amplifier, that is, when a positive AC signal is applied to the input terminal 1 connected to the base of the front drive transistor 10, the front drive transistor 10 becomes conductive, and the drive transistor 12
The base current flows into its collector, the drive transistor 12 becomes conductive, one of the SEPP-connected output transistors 21 becomes conductive, and its collector current is supplied to the load resistor 8, and the emitter of the drive transistor 12 is connected to the power supply terminal 2. A current is supplied from a voltage source applied to the input terminal 1 through the resistor 15 and the transistor 11. Also, when a negative AC signal is applied to the input terminal 1, the pre-drive transistor 10 is non-conductive, and the base of the drive transistor 19 is not conductive. A current is supplied from the bootstrap terminal 3 through the resistor 16 to make it conductive, and the other SEPP-connected output transistor 20 becomes conductive, and its emitter current is supplied to the load resistor 8. At this time, the potential of the output terminal 4 falls to around the ground potential when the output transistor 21 is conductive, and rises to around the power supply voltage when the output transistor 20 is conductive.
The potential of the bootstrap terminal 3, which is AC short-circuited by the bootstrap capacitor 6, also swings in both positive and negative directions with the same amplitude as that of the output terminal 4.

When the potential of the bootstrap terminal 3 is varied in both positive and negative directions in an alternating current manner as described above, for example, the drive transistor 19 and the output transistor 20
At the time of positive amplitude output when is conducting, the potential is generally higher than the potential of the power supply terminal 2, and the charge accumulated in the bootstrap capacitor 6 is applied to the base and collector of the drive transistor 19 as a base current and a collector current, respectively. and resistance 15
A current flows through the power supply terminal 2 and discharges it, and also, for example, the drive transistor 12 and the transistor 1
1 and the output transistor 21 are conducting, and the voltage is generally lower than that of the power supply terminal 2, and a charging current flows into the bootstrap capacitor 6 through the resistor 15.

In such a power amplifier, when the output terminal 4 swings in the negative direction, the emitter current of the drive transistor 12 and the charging current of the bootstrap capacitor 6 are flowing through the resistor 15 through the transistor 11.
The charging current is limited and transfers to the next output of the opposite polarity before it is sufficiently charged. The drive current at which the drive transistor 19 drives the transistor 20, which is determined by the charge accumulated across the bootstrap capacitor 6, is also limited, so that when the output terminal 4 swings in the positive direction, the output transistor 20
is not sufficiently conductive, and the output waveform of output terminal 4 changes from the positive amplitude to the negative amplitude A, as shown in waveform a in Figure 2A.
The smaller positive and negative waveforms become asymmetrical,
Furthermore, the maximum output amplitude becomes narrower.

One way to solve the above drawback is to set the resistance value of the resistor 15 to a small value to increase the charging current of the bootstrap capacitor 6. However, at this time, the charge accumulated across the bootstrap capacitor 6 increases, causing the drive transistor 19 and the output transistor 2 to increase.
At the instant of the positive cycle when 0 conducts, the output transistor 20 is driven through the drive transistor 19 by the charge stored in the bootstrap capacitor 6, and the potential of the output terminal 4 rises to near the potential of the power supply terminal 2. However, since the resistance value of the resistor 15 is small, the discharge time constant of the charge stored in the bootstrap capacitor 6 is short, and therefore the drive transistor 19 and the output transistor 2
During the positive conduction cycle period of 0, the drive current of the output transistor 20 is limited, and the potential of the output terminal 4 drops from near the potential of the power supply terminal 2. At this time, the output waveform of the output terminal 4 is as shown in FIG. The waveform on the positive side is missing, as in

As described above, in conventional power amplifiers, the positive and negative half-cycle waveforms are asymmetrical, resulting in an output waveform with a lot of distortion with respect to the input signal, as well as an output amplitude with a large voltage loss for a given applied power supply voltage. Therefore, the power obtained by the load resistor 8 also becomes small.

As a measure to prevent such output asymmetry,
It is also conceivable to connect the power supply terminal 2 to the bootstrap terminal 3 through a cascade connection of a forward diode (not shown) and a resistor 15. in this case,
Since the charge accumulated in the bootstrap capacitor 6 blocks the current flowing to the power supply terminal 2 side, the time constant for discharging the charge becomes long, making this an effective means. However, it is extremely inconvenient to drive the power amplifier at a low power supply voltage (here, a voltage around 3 to 5 V). This is because, in the power amplifier, in order for the upper transistor, that is, the drive transistor 19 and the output transistor 20, to become conductive, the base-emitter forward voltage (approximately 0.7V) of the transistors 19 and 20 and the base current of the drive transistor 19 are required. A voltage difference greater than or equal to the sum of the voltage drop across the resistor 16, the forward diode (not shown) connected between the power supply terminal and the bootstrap terminal, and the voltage drop across the resistor 15 occurs between the power supply terminal 2 and the output terminal 4. This is because it is necessary to apply a bias to the power amplifier, and in this power amplifier, there is a voltage loss corresponding to the voltage drop of the forward diode connected in series with the resistor 15 between the power supply terminal 2 and the bootstrap, which causes a large voltage loss due to low power supply voltage operation. It becomes a drawback.

Furthermore, in the conventional power amplifier shown in FIG. 1, a current equal to the sum of the charging current for charging the bootstrap capacitor 6 and the collector current of the transistor 11 flows through the resistor 15, so that the charging voltage of the bootstrap capacitor 6 is lost. In order to prevent this, the means of directly connecting the collector of the transistor 11 to the power supply terminal 2 is an effective means for improving the loss of charging voltage to the bootstrap capacitor 6. However, in such a power amplifier, when the output terminal 4 swings near the power supply voltage, the base voltage of the transistor 11 swings higher than that, so that the base-collector of the transistor 11 is forward biased, and the transistor 11 Almost no current flows through the emitter of the transistor, making it impossible to drive the drive transistor 12 and the output transistor 21 sufficiently. This is because when a capacitive load or the like is connected between the output terminal 4 and the ground terminal 5, the output terminal 4 swings close to the power supply voltage, the output transistor 21 becomes conductive, and the load cannot be driven, resulting in a low output. It has the disadvantage that it partially blocks it.

In view of the above-mentioned drawbacks of the conventional technology, it is an object of the present invention to provide a symmetrical half-cycle output waveform in both positive and negative directions with a simple circuit configuration, as well as an output waveform with a large maximum output amplitude, and to provide a power supply suitable for semiconductor integrated circuits. The goal is to obtain an amplifier.

Another object of the present invention is to obtain a power amplifier that can be driven with a low power supply voltage.

A further object of the present invention is to provide a power amplifier that can provide sufficient output even with a capacitive load.

According to the present invention, first and second output transistors of the same polarity are connected in series between their respective collectors and emitters with respect to a DC power supply, and the The third output transistor has the same polarity as the first output transistor.
A collector is connected to the bases of the drive transistor and the second output transistor, a fourth drive transistor having a different polarity from the second output transistor, and a base of each of the third and fourth drive transistors. first and second resistors connected in series between a common connection point and one end of a power supply terminal; a connection point between the first and second resistors; an emitter of the first output transistor; and the second output. a bootstrap capacitor provided between the output terminals connected to the connection point of the collector of the transistor; and an emitter connected to the emitter of the fourth drive transistor;
a fifth biasing transistor whose base is connected to the connection point of the first and second resistors through a third resistor;
The emitter is at the collector of the bias transistor, and the base is at the connection point of the first and second resistors.
A power amplifier is obtained in which a sixth transistor is inserted into one end of a power supply terminal, the collector of which is connected in a direct current manner.

The present invention will be described in detail below with reference to the drawings.

FIG. 3 shows an embodiment of the present invention, in which the same parts as in FIG. This is achieved by adding transistors 23 whose bases are connected to each other and whose collectors are connected through diodes 22 whose anodes are connected to the power supply terminal 2. When the output terminal 4 and the bootstrap terminal 3 swing downward and the output transistor 21 becomes conductive to drive the load resistor 8, the output transistor 21
The drive base current is supplied from the voltage source applied to the power supply terminal 2 to the diode 22, the transistor 23, the bias transistor 11, and the drive transistor 1.
Since the base current of the transistor 23 is supplied from the power supply through the resistor 15, the current flowing through the resistor 15 as the drive current of the output transistor 21 is approximately equal to the collector current of the bias transistor 11, that is, the emitter current of the transistor 23. The grounded emitter forward current amplification factor h is 1/ _FF . The current amplification factor _hFF of a transistor integrated into a semiconductor circuit is usually 30~
Since it is a large value of 300, it is a very small value of 1/30 to 300 compared to the conventional example shown in Figure 1.
The voltage drop generated by flowing through the resistor 15 is also small, which means that the voltage loss when charging the bootstrap capacitor 6 is almost negligible, and therefore the charge accumulated in the bootstrap capacitor 6 also increases significantly. As a result, the drive transistor 19 and the output transistor 20 driven by this charge can sufficiently drive the load resistor 8. At this time, the output waveform outputted to the output terminal 4 has a larger amplitude than the conventional a waveform, such as the b waveform in FIG. 2A, and is symmetrical in that the upper and lower amplitudes are equal. The diode 22 is for preventing current from flowing from the base of the transistor 23 to the power supply terminal 2 through the collector when the bootstrap terminal 3 swings upward and becomes higher than the voltage at the power supply terminal 2.

In the power amplifier described above, there is no disadvantage in operation at a low power supply voltage, and furthermore, even in capacitive loads etc., the collector of the bias transistor 11 is connected from the bootstrap terminal 3 to the transistor 23.
Since current is supplied through the base and emitter of the output transistor 21, the load side can be driven without the output transistor 21 being cut off.

As explained above, the power amplifier of the present invention has a simple circuit configuration and can provide an output waveform with a vertically symmetrical half-cycle and a large maximum output amplitude.

Further, according to the present invention, it is possible to realize power amplification that can operate at a low power supply voltage and provides a normal output even with a capacitive load.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a conventional power amplifier, waveforms a and b in FIG. 2A are output waveforms of the output terminal 4 according to the conventional example, and waveform b in FIG. FIG. 3 is an output waveform diagram of the output terminal 4 according to an example, and FIG. 3 is a circuit diagram showing an embodiment of the present invention. 3...Bootstrap terminal, 4...Output terminal, 6...Bootstrap capacitor, 19,
12... Drive transistor, 20, 21... Output transistor, 15, 16... Load resistor, 11...
...Bias transistor, 23...transistor, 22...diode.

Claims (1)

[Claims] 1 A unipolar first transistor having a collector-emitter conductive path connected between a first power terminal and an output terminal, and a collector-emitter conductive path connected between the output terminal and a second power terminal. the unipolar second transistor having an emitter connected to the base of the first transistor; and the unipolar third transistor having a collector connected to the base of the second transistor. 4
means for connecting the bases of the third and fourth transistors; first and second resistors connected in series between the base of the third transistor and the first power supply terminal; a bootstrap capacitor provided between the connection point of the first and second resistors and the output terminal;
the unipolar fifth transistor, the emitter of which is connected to the emitter of the transistor, and the base of which is connected to the connection point of the first and second resistors via a third resistor; and the collector of the fifth transistor. an emitter is connected to the first resistor, a base is connected to the connection point of the first and second resistors, and the first
A power amplifier comprising the unipolar sixth transistor whose collector is connected to the power supply terminal of the transistor through a diode.