Bipolar Junction Transistor Circuits:IC Biasing Using Current Mirrors.

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IC Biasing Using Current Mirrors

Differential stages are very important in IC amplifier design. These stages require a constant DC current for proper bias. The diode-biased current sink or current mirror of Figure 10.9 is a popular method of creating a constant current bias for differential stages and other stages requiring a constant current bias.

The concept of the current mirror was developed specifically for analog IC biasing and is a good example of a circuit that takes advantage of the excellent matching characteristics that are possible in ICs. In the circuit of Figure 10.9, the current I2 is intended to be equal to or “mirror” the value of I1. Current mirrors can be designed to serve as sinks or sources.

The general function of the current mirror is to reproduce or mirror the input or reference current to the output while allowing the output voltage to assume any value within some specified range. The current mirror can also be designed to generate an output current that equals the input current multiplied by a scale factor K. The output current can be expressed as a function of input current as

Bipolar Junction Transistor Circuits-0021

where K can be equal to, less than, or greater than unity. This constant can be established accurately by relative device sizes and will not vary with temperature.
Current Source Operating Voltage Range

Figure 10.10 shows an ideal or theoretical current sink in (a) and a practical sink in (b). The voltage at node A in the theoretical sink can be tied to any potential above or below ground without affecting the value of I. In contrast, in the practical circuit of Figure 10.10(b), it is necessary that the transistor remains in the active region to provide a current of

Bipolar Junction Transistor Circuits-0022

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where BVCE is the breakdown voltage from collector to emitter of the transistor. This voltage range over which the current source operates is called the output voltage compliance range or the output compliance.
Current Mirror Analysis

The current mirror is again shown in Figure 10.11. If devices Q1 and Q2 are assumed to be matched devices, we can write

Bipolar Junction Transistor Circuits-0024

where VA is the Early voltage. In effect, the output stage has an output impedance given by Eq. (10.16). The current mirror more closely approximates a current source as the output impedance becomes larger.

Bipolar Junction Transistor Circuits-0025

If we limit the voltage VC2 to small values relative to the Early voltage, IC2 is approximately equal to IC1. For IC designs, the voltage required at the output of the current mirror is generally small, making this approximation valid.

The input current to the mirror is larger than the collector current and is

Bipolar Junction Transistor Circuits-0026

For typical values of b these two currents are essentially equal. Thus, a desired bias current, IOUT, is generated by creating the desired value of IIN.

The current IIN is normally established by connecting a resistance, R1, to a voltage source VCC to set

Bipolar Junction Transistor Circuits-0027

where N is the number of output devices.

The current sinks can be turned into current sources by using pnp transistors and a power supply of opposite polarity. The output devices can also be scaled in area to make IOUT larger or smaller than IIN. Figure 10.13 shows a multiple output mirror where output source currents I2 and I3 are referenced to input current I1 as also are sink currents I4 and I5. If all device sizes are equal, then I3 = 2I2 and I5 = 2I4.

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Current Mirror with Reduced Error

The difference between output current in a multiple output current mirror and the input current can become quite large if N is large. One simple method of avoiding this problem is to use an emitter follower to drive the bases of all devices in the mirror as shown in Figure 10.14.

The emitter follower, Q0, has a current gain from base to collector of b + 1 reducing the difference between IO and IIN to

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The Wilson Current Mirror

In the simple current mirrors discussed, it was assumed that the collector voltage of the output stage was small compared with the Early voltage. When this is untrue, the output current will not remain constant,

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but will increase as output voltage (VCE) increases. In other words, the output compliance range is limited with these circuits due to the finite output impedance of the BJT. A modification of the improved output current mirror of Figure 10.14 was proposed by Wilson [10] and is illustrated in Figure 10.15.

The Wilson current mirror is connected such that VCB2 = 0 and VCB1 = VBE0. Both Q1 and Q2 now operate with a near-zero collector–emitter bias even though the collector of Q0 might feed into a high voltage point. It can be shown that the output impedance of the Wilson mirror is increased by a factor of b /2 over the simple mirror. This higher impedance translates into a higher output compliance. This circuit also reduces the difference between input and output current by means of the emitter follower stage.

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