1. Field of the Invention
The present invention relates to an output circuit to be used in an amplifier or the like, and relates, more particularly, to an output circuit with an enlarged output dynamic range.
2. Description of Related Art
In general, an amplifier has built therein an input circuit for inputting a signal from the outside of the amplifier and an output circuit for outputting a signal to the outside of the amplifier, and the like. FIG. 1 is a circuit diagram for showing a conventional output circuit.
The conventional output circuit is provided with a differential amplifier circuit portion 11 having a differential amplifier circuit for producing an in-phase signal and an opposite-phase signal with respect to an input signal, and an output-stage circuit portion 12 having output-stage transistors T21 and T22 with multi-stage connections.
The differential amplifier circuit portion 11 has transistors T23 and T24 differentially connected to each other. The base of the transistor T23 is connected to an input terminal of the output circuit. To the collectors of the transistors T23 and T24, resistor elements R23 and R24 re connected respectively. Further, to the emitters of the transistors T23 and T24, resistor elements R25 and R26 are connected respectively. The resistor elements R25 and R26 are connected with a constant current source I21 for generating a constant current. The base of the transistor T24 is connected to the connection point of resistor elements R27 and R28 connected in series.
In the output-stage circuit portion 12, an output terminal of the output circuit and a resistor element 21 are connected to the emitter of the output-stage transistor T21. A resistor element R22 is connected to the emitter of the output-stage transistor T22. The base of the output-stage transistor T21 is connected to a connection point "c" between the transistor T23 and the resistor element R23, and the base of the output-stage transistor T22 is connected to a connection point "d" between the transistor T24 and the resistor element R24.
A positive power is supplied to the resistor elements R23, R24 and R27 and the collectors of the output-stage transistors T21 and T22. The constant current source I21 and the resistor elements R21, R22 and R28 are grounded.
When an input signal of an amplitude V21 is supplied to the conventional output circuit having the abovedescribed structure, this input signal is amplified into output signals with amplitudes V22 and V22' respectively by the amplification effect of the transistors T23 and T24. In this case, the output signal V22 becomes in opposite phase to the input signal, and the output signal V22' becomes in-phase with the input signal, so that the emitter potential of the output-stage transistor T21 becomes in the opposite phase.
However, there is a problem that the output dynamic range of the above-described conventional output circuit is constrained by a collector-emitter voltage V.sub.CE1 of the transistors T23 and T24 within the differential amplifier circuit portion 11 and by a collector-emitter voltage V.sub.CE2 of the output-stage transistor T21 within the output-stage circuit portion 12.
In other words, the output dynamic range of the conventional output circuit becomes V22.ltoreq.r23.times.i21 when the resistance value of the resistor element R23 is expressed as r23 and the current value of the constant current value I21 is expressed as i21. However, this output dynamic range is constrained by the collector-emitter voltages of the transistors T21, T23 and T24 as shown by the following formula 1. EQU V22.ltoreq.r23.times.i21&lt;V.sub.CE1 &lt;V.sub.CE2 (1)
Therefore, for example, when the potential of the connection point "c" becomes low, the emitter potential of the output-stage transistor T21 also becomes low. Thus, the margin of the collector-emitter voltage V.sub.CE2 of the output-stage transistor T21 becomes the minimum. As the emitter potential and the collector potential of the output-stage transistor become opposite phase components, the output dynamic range is constrained by the performance (collector-emitter voltage) of the transistor.