Many portable circuits require precision measurement capability along with low power operation to minimize current and battery drain. When searching for an amplifer with these requirements, it may be relatively easy to meet the low power and precision requirements, but the drive current may not be adequate for these low power devices. For example, looking at one of Linear Technology's recently announced low power precision amplifiers, the LT6020 is a dual, precision amplifier with 30µV offset (max at 25°C), low 0.5uV/°C (max) drift, low power (100µV/amplifier), 400kHz bandwidth and an impressive 5V/µs slew rate. With the output short-circuited, it provides 5.5mA of currrent to the load, which may not be enough for driving such applications as a solenoid, valve, multi-pole filter, or other heavy loads. Also keep in mind that as load current increases, the open loop gain decreases, which can adversely affect the device's accuracy and a circuit's performance.
To provide more current a simple push-pull transistor output stage can be added to increase the amplifiers drive capability.
In this circuit, we add the 2N3904 and 2N3906 NPN/PNP transistor pair in a push-pull configuration to increase output current. The 2N3904 40V, 200mA NPN transitor with a minimum beta (ß) gain of 80 (IC = 1mA, VCE = 1V). The 2N3906 is the complementary -40V, 200mA PNP transisitor also with a minimum beta (ß) gain of 80 (IC = -1mA, VCE = -1V). The 1k resistor between the transistor bases and emitters allows the LT6020 to provide the first 600µA of load current before the NPN and PNP start providing additional current. This resistor improves linearity in the middle of the range as the transistors turn off and on.
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