Amplifiers – With semiconductor amplifying device – Including differential amplifier
Patent
1980-11-09
1983-08-02
Mullins, James B.
Amplifiers
With semiconductor amplifying device
Including differential amplifier
330260, H03G 330, H03F 345
Patent
active
043968910
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a gain control circuit suitable for implementation in a monolithic device and substantially intended to be used in line and microphone amplifiers for telephone instruments.
DESCRIPTION OF PRIOR ART
It is known to use differential amplifier stages in gain control circuits intended to be included in amplifiers designed in an integrated circuit. By means of these stages it is possible to design temperature stable amplifiers. Gain control circuits of this type are for example described in the U.S. Pat. No. 3,684,974 or in the German Pat. No. 2 262 580.
A gain control circuit according to the above mentioned patents operates according to the current sharing principle implying that the signal current is shared by one or more differential stages, the shares being determined by a control current. The controlled signal voltage is taken out across one (or more) load impedances where the current share determined by the control voltage is transformed into an output voltage.
SUMMARY OF THE INVENTION
The gain control circuits previously known are intended as volume controls in radio sets and audio amplifiers. Therefore, they must have a large control range whereas the total gain and a possible drift of the gain is of minor importance. It is also obvious that gain control circuits of this type cannot be used in telephone instruments with their great demands on the nominal attenuation as well as of the control range. The tolerated deviation from the specified attenuation value is of magnitude .+-.0.1 dB when the control range is 6 dB.
An object of the present invention is to achieve a gain control circuit which fulfils the above requirements by including at least a first and a second transistor differential amplifier whose inputs are connected in parallel to a signal source and (the characteristics of the invention appear from the appended claims). A gain control circuit according to the invention thoroughly fulfils the demands for a defined amplification required in an amplifier for telephone purposes at the same time as low noise, low distortion and low power consumption are obtained.
BRIEF DESCRIPTION OF THE DRAWING
The invention will be more fully described with reference to the accompanying drawing, where: unbalanced output; and
PREFERRED EMBODIMENTS
The circuit shown in FIG. 1 includes a first differential stage with the transistors T1 and T2, a second differential stage with the transistors T3 and T4 and a third differential stage with the transistors T5 and T6. A signal is applied across the input terminals 11 and 12. The input 11 is connected through the resistor R1 to the base of the transistor T1 and through the resistors R1 and R2 to the base of the transistor T4. The base electrodes of the transistors T2 and T3 are interconnected and connected to the input 12 and the output 14. The collectors in the transistors T1 and T4 are connected to a constant current source I1 and to an input of an amplifier F while the collectors of the transistors T2 and T3 are connected to a constant current source I2 and the second input of the amplifier F. The interconnected emitters of the transistors T1 and T2 are connected to the collector of the transistor T5 in the third differential amplifier. The collector of the transistor T6 in the same differential amplifier is connected in the same way to the emitters of the transistors T3 and T4. The emitters of the transistors T5 and T6 are connected to a constant current source Is. The base electrodes of the transistors T5 and T6 are connected to a DC control voltage Uc. There is a negative feed back from the output of the amplifier F through the resistor R3 to the base circuit of the transistor T4 (directly) and the transistor T1 (through R2). Thus the differential amplifiers T1, T2 and T3, T4 have different negative feed back factors.
The circuit operates in the following way. The sharing of the constant current Is between the transistors T5 and T6 can be controlled through the DC voltage Uc. This implies that the
REFERENCES:
patent: 3452289 (1969-06-01), Ryan
Berg Bengt O.
Johansson Jan H.
Mullins James B.
Telefonaktiebolaget L M Ericsson
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