Current mirror with improved output impedance at low power supplies
A current mirror circuit arrangement is formed to maintain a high output impedance when utilized with a relatively low voltage power supply. A common mode voltage regulator circuit is utilized in conjunction with the output branch of the current mirror to eliminate the need for an additional active device in series with the output transistor of a current mirror to control its drain voltage. The elimination of the second active device thus increases the available headroom for the output branch (i.e., adds one V DS ). The increased headroom in the inventive current mirror is particularly advantageous for low voltage applications, since it is capable of maintaining the high output impedance required for accurate mirroring of the input current.
1 . A current mirror comprising
an input circuit branch including at least one reference transistor, defined as including a control input, and an input current (I in ) applied to the at least one reference transistor;
an output circuit branch including a single transistor with a control input; and
a differential control circuit coupled between a bias voltage source (V B ) and the input of the single transistor forming the output circuit branch, the differential control circuit configured to adjust the common mode voltage so as to maintain the voltage across the single transistor (V OUT ) at an essentially constant value.
2 . A current mirror as defined in claim 1 wherein the input circuit branch comprises a pair of reference transistors.
3 . A current mirror as defined in claim 2 wherein at least one reference transistor of the pair of reference transistors is coupled to exhibit diode properties.
4 . A current mirror as defined in claim 1 wherein the transistors comprise bipolar devices.
5 . A current mirror as defined in claim 1 wherein the transistors comprise MOS devices.
6 . A current mirror as defined in claim 1 wherein the differential control circuit further comprises
an amplifier coupled at a first input to the bias voltage source and at a second input to the single transistor of the output circuit branch, the output of the differential amplifier representing the difference between the bias control voltage V B and V OUT ;
a regulator coupled to the output of the differential amplifier for regulating the common mode voltage applied to the input of the output circuit branch single transistor; and
a common mode circuit responsive to the output of the regulator for providing a pair of common mode output signals.