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Active Loads

Using an Active Load

A typical NPN BJT amplifier stage. Adding an active load to the BJT amplifier.

The circuit to the left is the schematic diagram of a typical small-signal amplifier stage. In this circuit, R1 and R2 set the base bias voltage, while RE provides stabilizing negative feedback and sets the emitter current, IE. C1 and C2 serve to couple ac signals into and out of the circuit without disturbing the dc bias conditions established by the resistors. Finally, RL, the collector load resistor, is chosen to set the collector voltage, VC, to VCC/2 or just slightly higher. This allows the collector voltage to vary as much as possible when an ac signal is applied to the input.

The problem is that RL must be large to get a high voltage gain from this stage, but it must be small if IC is to be very high. For example, if VCC = 12 V and we want IC = 1 mA, then RL must be 6K. We can reduce that to 5.6K or raise it to 6.2K in order to use a standard value, but this makes little difference — this is a small load resistance to use to develop a high signal voltage at the collector. We could reduce the collector current to enable a higher value of RL, but if we go too far in that direction, we starve the transistor of usable current. Besides, if we try building the circuit into an IC, a large resistance requires a lot of space on the die.

A very practical solution is to use something like the circuit to the right. Here, we have replaced RL with a current mirror, using Q2 as an active load for Q1. Now, the effective value of RL is ROUT of the mirror, which for this circuit is rO ≈ VA/IC = 50K, assuming VA = 50 V and IC = 1 mA. In addition, the dc bias voltage for VC is no longer set by the value of RL. This means that C2 might not be necessary, provided the next stage can share biasing with this one.

Advantages and Disadvantages

There are as usual trade-offs between the use of a resistive load and an active load in an amplifier circuit of this kind. A large part of that has to do with the fact that this circuit can be built either with discrete components or as an IC.

If we build this circuit using discrete components, the use of a collector load resistor is cheap. One resistor costs no more than another, but adding two transistors to the circuit adds significant cost. (Even if each transistor costs 25¢, that's 50¢ per circuit. If we're mass-producing the product, the cost becomes major.) In addition, discrete BJTs will work as a current mirror, but don't match well. This means that mass-produced circuits won't all behave alike. Matching discrete transistors is expensive. Therefore, it will probably be cheaper and easier to use just RL and add another stage if higher gain is required.

If we manufacture this circuit as part of an IC, we find that the size of a component on the die contributes greatly to cost. In other words, space or real estate on the die is at a premium. High-value resistors take a lot of space. Also, the mirror transistors will be manufactured side-by-side on the die, at the same time and under the same conditions, so they will match very closely. For mass production, it won't matter if the parameters vary a bit from die to die, since the mirror current will be set by RREF and VCC. As part of an IC, the circuit will likely be cheaper using the active load. Circuit performance will also be better with the higher load resistance on Q1.

Another consideration is that we might well use Q3 as the mirror reference for multiple active load transistors in a multi-stage amplifier circuit.

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