Current sources made by using active devices have come to be widely used in analog integrated circuits for both biasing elements as well as load devices for amplifier stages. The use of current sources in biasing can result in superior insensitivity of circuit performance to power supply variations and to temperature. When used as a load element in transistor amplifiers, the high incremental resistance of the current source results in high voltage gain at low power-supply voltages.
Current mirror circuits are typically used for generating an accurate large current from a small reference current. Current mirror circuits that operate at low supply voltages must be able to generate the accurate larger current when the supply voltage is less than twice a base emitter voltage drop (Vbe) for a bipolar transistor or when the supply voltage is less than twice a threshold voltage (Vt) for a FET device. These low supply voltages can occur in devices that operate using two 0.9V battery cells, such as mobile terminals.
A prior art patent, EP 1 213 636 describes a current mirror circuit. Unfortunately, this circuit does not allow wide ratio operation and is more difficult to stabilize. The current ratio is limited to the ratio of the NPN devices used. Loop stability is harder to achieve since it is difficult to achieve one dominant loop pole. Stability in the current mirror is known to those of skill in the art to be an important quality for the current mirror circuit because of the potential to introduce oscillations that disrupt the operation of the load circuit. Wide ratio current mirrors are often used with power amplifier circuits and the wide ratios required by these PAs are not attainable by other means. Utilizing a low ratio current mirror would result in appreciable wasted current consumption by virtue of the fact that the bias circuits would have to provide much greater input currents. In other words, the current being ‘mirrored’ becomes a more significant fraction of the current being supplied to the load. When the mirror output transistor is modulated with a large amplitude RF signal another disadvantage of this scheme becomes apparent, particularly where a large mirror ratio is chosen. The RF signal causes an increase in the mean collector current in the modulated transistor and therefore an increase in the mean base current. This is because the transistor is biased into class B operation. This mean base current is sourced from the current mirror not from the RF source and is usually significantly larger than the quiescent component. Thus a large RF signal can reduce the effective mirror ratio.
Other solutions that incorporate operational amplifiers (OpAmps) are also known to those of skill in the art, however these circuits are quite complex and often less accurate. An Opamp, on its own, is inherently more complex than simple transistor circuits. OpAmp circuits are subject to voltage offset, which can be an issue when we are dealing with NPN current mirror circuits where a few mV represents a significant error term. They typically are not able to operate at 1.5V. Other conventional current mirror circuits also have difficulties operating with large ratios.
A need therefore exists for a wide ratio current mirror for offering stable operation at low supply voltages. It is therefore an object of the invention to provide a wide ratio and low voltage current mirror that offers stability at low supply voltages. It is a further object of the invention to provide low voltage current mirror for use with a differential amplification stage.