1. Field of the Invention
This invention relates generally to transmitters used in telephones systems and, more particularly, to an electric circuit for matching an electret microphone to the electrical characteristics of existing carbon compatible telephone networks.
2. Description of the Related Art
Carbon compatible telephone networks were originally designed for use with telephones employing older technology microphones. That is, microphones with carbon pellets. This type of microphone provides a poor quality of transmission, due primarily to problems associated with the carbon pellets. For example, the pellets often unpack or are packed unevenly. Further, carbon microphones are relatively expensive to manufacture.
More recently, microphone technology has advanced to a more modern state with the development of electret microphones. Electret microphones provide high quality transmission signals at a relatively low cost, with the added benefit of being much smaller in size than the carbon microphones. In other words, electret microphones are particularly well suited for use in the telephone industry.
However, the carbon compatible networks originally designed for use with carbon microphones do not directly interface with telephones employing electret microphones. The carbon compatible networks have a low input impedance, while the electret microphone has a high impedance. Thus, when electret microphones are connected to carbon compatible networks, impedance mismatches occur.
Previously, it has been common practice for handsets employing electret microphones to be interfaced with carbon compatible networks via the use of Darlington pair amplifiers. These Darlington amplifiers are designed to overcome problems arising from mismatched impedances. However, these solutions introduce new problems. The Darlington type amplifiers are designed around bipolar transistors. The emitter of the first transistor is connected to the base of the second transistor, and both collectors are tied together. This connection scheme produces an equivalent transistor with a high gain, which is desirable. Unfortunately, it also results in an equivalent transistor with a base-emitter voltage drop twice that of a normal bipolar transistor. Hence, the demands on bias voltage are undesirably increased.
This last effect is highly undesirable, since the available bias voltage is relatively low. Long transmission lines or additional loads limit the available voltage range even further, and often result in poor transmission quality. For example, in a telephone system that employs multiple extensions, it is common for the transmission quality to degrade substantially when more than one phone is operated on the same circuit. Operating two phones simultaneously outstrips the capacity of the central office to provide voltage to both extensions.
Additionally, a rectifier is commonly used to establish a DC supply voltage for biasing purposes. Both the electret microphone and the amplifier require biasing for stable operation. Presently, standard implementations use silicon rectifiers that exhibit a relatively large voltage drop. As mentioned before, large voltage drops are undesirable in that they degrade the transmission quality and in some instances even limit the use of the handset.
The present invention is directed to overcoming one or more of the problems set forth above.