1. Field of the Invention
The present invention is concerned with a direct-coupled full stage cascaded amplifier having a pre-drive stage, a drive stage and an output stage, all of which stages being direct-coupled to each other. More specifically, the present invention pertains to an OCL (output capacitorless) amplifier of the type that FET's are used as the amplifier elements and that all of these stages are directly coupled to each other. This amplifier of the present invention can be used widely for many purposes such as amplification of audio signals, direct current amplification and pulse amplification.
2. Brief Description of the Prior Art
An FET has many excellent features such as presenting a high input impedance, a low noise index, a high gm (trans-conductance) and a high frequency characteristic, and accordingly, it has a quite superior utility as an amplifier element. Moreover, the FET has another feature that it is hardly affected by ambient temperature as compared with a bipolar transistor. As such, an FET (field effect transistor) is quite advantageous when used as an amplifier element especially when it is used as a component of a direct-coupled full stage cascaded amplifier to which changes in bias voltage are very undesirable.
Especially, the recent development of power FET's (vertical type FET's) has accelerated the use of power FET's in the field of power amplifier of audio signals.
In the past, however, a direct-coupled full stage cascaded amplifier utilizing FET's has been constructed by following the circuit arrangement of the direct-coupled full stage amplifier utilizing bipolar transistors. Accordingly, it has not been possible to make full use of the unique features of FET's. Furthermore, according to the known art, the circuit arrangement, especially the arrangement of the biasing circuit, has been complicated, and such a circuit arrangement of the prior art has been susceptible to the effect of the ambient temperature, and what is more, the control of the bias voltage has been difficult to make. In view of these aspects, it has been extremely difficult to materialize the so-called OCL amplifier without an output capacitor, in the form of direct-coupled full stage cascaded amplifier arrangement.