Amplifiers – With semiconductor amplifying device – Including combined diverse-type semiconductor device
Patent
1984-05-18
1986-01-28
Mullins, James B.
Amplifiers
With semiconductor amplifying device
Including combined diverse-type semiconductor device
330289, H03F 316
Patent
active
045674450
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to an amplifier circuit with high input impedance and low output impedance, preferably intended as a microphone amplifier in telephone apparatus with electret microphones and for being supplied with current via the telephone line.
BACKGROUND ART
Electret microphones are used to an ever-increasing extent in modern telephone apparatus, since they give good sound quality, have low power consumption and are inexpensive in manufacture. As with all electrostatic microphones, the electret microphone has a very high impedance and low output power, so an amplifier must therefore be connected between the microphone and the line. Due to the electret microphone's high impedance, this amplifier should be immediately adjacent the microphone to avoid circuit noise. Suitably, the microphone and amplifier are built together to a capsule which is placed in the telephone handset. Current supply then suitably takes place using the same conductor as for the signals from the handset to the telephone set. A suitable circuit for connecting a source having very high impedance to a load with low impedance suitably contains an input stage with a field-effect transistor directly connected to one or more emitter follower stages. Such circuits are known in the literature e.g. from "Electronics" Feb. 28, 1972, page 80, which illustrates an amplifier for a capacitive transducer.
DISCLOSURE OF INVENTION
When an amplifier of this kind is to be used as a microphone amplifier, large demands are placed on it for operation at low voltages (lower than 2 volts) and within a large temperature range (-20.degree.-+70.degree. C.) with low distorsion.
For the field-effect transistor to work linearly at constant bias over the whole temperature range the bias must be made high, due to the threshold voltage increasing with the temperature. At low temperatures, the sum of this voltage and the voltage, increasing with falling temperature, of the base-emitter voltage in the emitter follower stage will then be so large that there is the risk of the voltage of the amplifier current supply circuit not being sufficient. The field-effect transistor has, however, the property that it does not need such a high voltage at low temperatures, due to the threshold voltage having a positive temperature coefficient.
In accordance with the invention, this relationship can be utilized for optimally utilizing available supply voltage in an amplifier circuit with one input stage having at least one field-effect transistor directlyg connected to a bipolar amplifier stage. The characterizing features of the invention are disclosed in the accompanying claims.
DESCRIPTION OF FIGURE
The invention will now be described in detail in conjunction with the appended drawing whose sole FIGURE, illustrates a circuit diagram for an electret microphone amplifier.
PREFERRED EMBODIMENT
An integrated amplifier, denoted by the numeral 11 in the sole FIGURE, has only three exterior connections, namely 12, which is a common signal output and positive voltage supply, a common point 13 for the amplifier input and output (earth) as well as negative voltage supply, and the signal input 14. An electret microphone M is connected between the signal input 14 and earth 13. The integrated circuit is built up conventionally with a p substrate and an epitaxial n layer, wherein transistors and other elements have been made using the diffusion technique.
The input stage of the amplifier consists of a field-effect transistor Q12 of the P channel type, with its gate connected to the input 14. This transistor Q12 is fed with current from a current mirror circuit including the transistors Q1, Q2 and the field-effect transistor Q3. The field-effect transistor Q3, which is exactly equal to the field-effect transistor Q12, is connected as a constant current generator supplying the diode-connected transistor Q1. The current in transistor Q1 is reflected to the identically similar transistor Q2 driving the same current through the field-effect transistor Q12. The tra
REFERENCES:
patent: 4482868 (1984-11-01), Whatley
Mullins James B.
Telefonaktiebolaget LM Ericsson
Wan G.
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