In the current mirror shown in FIG. 1100 transistor Q1103 accepts input current Iin and is connected to transistor Q2105 whereby the transistor output current Iout is related in magnitude and direction to Iin by the transconductance of the transistors. This is due to the common voltage from base to emitter (VBE) of the transistors Q1 and Q2 as connected. When used with transistors, and especially in a circuit using bipolar transistors, the relationship between the input current to Q1103 and output current received by Q2105 may be precisely controlled.
In the prior art, both transistors in a current mirror as shown in FIG. 2 have a common emitter connection referred to as the reference node, which in the case of bipolar transistors is commonly the emitter nodes or through resistors in the emitter nodes of the transistors. The VBE of the input transistor with respect to the reference node controls the VBE of the output transistor with respect to the reference node.
It is often desirable that the reference node of the output transistor be connected to some other point than the reference node of the input transistor. This can be a requirement, for example, for an operational amplifier, where mirroring is used extensively, but the output may be referenced to a different voltage than the input reference; that is, there is some degree of isolation between input and output.
In an embodiment of the prior art shown in FIG. 2200, transistor 203, an NPN bipolar transistor in this embodiment, accepts an input current Iin and sets a VBE reference for transistor 207. Optional transistor 205 prevents excessive loading of the base connections of 205 and 207, but is often not included. The output from transistor 207 is received by transistors 209 and 211, wherein transistor 211 is optional, as discussed before. In like manner, transistor 213 provides an output current to transistors 215 and 217, and the final output current lout is supplied by transistor 219. Note that transistors 209, 211, and 213 are PNP transistors, since alternating transistor polarity types between stages, where transistors 203, 205, transistors 207, 209, 211, and 213, and transistors 215, 217, and 219 are all considered stages, simplifies interstage connections. The addition of the stages needed to avoid a common reference creates a problem in transistor matching, since errors in matching may occur in any stage, and are multiplied by subsequent stages. This can be a problem with respect to emitters, especially with respect to the emitters of PNP current mirrors such as transistors 209 and 213 in the current mirror composed of transistors 209, 211 and 213. PNP emitters generally both have a higher impedance and have more impedance across a contact, for example, from a PNP emitter to a connection such as a wire. It is very important where designing for matched impedance is a problem, that differences in emitter impedance, and especially resistance, be minimized.
What is needed is a bias circuit for precise control of the DC current with different reference nodes which is tolerant of emitter impedance effects in the current mirrors, and more particularly to allow an unbalanced impedance in the emitter circuit of the output transistor.