Certain electronics application requires high current to drive loads. Op-amp or operational amplifier usually have low output current. For example the uA741 or LM741 op-amp has output current capability of just 25mA. This maybe adequate for most application but sometimes we need more current at the output. There exist power op-amp like LM675 or LM12 but there is another way to boost the output current of ordinary operational amplifier. In this article it is shown how one can boost the output current of ordinary operational amplifier.
The output current of an op-amp can be increased using power transistors at the output of the op-amp. Such circuit are known as current booster. The power transistor to be used should have higher current gain and rating than the op-amp. Some popular NPN bipolar power transistors are 2N3055, TIP120, TIP122, BD139, IRF540, MJ15003, MJE3055 etc.
Unidirectional current booster
The following shows circuit diagram of unidirectional current booster implemented with 2N3055 npn bipolar power transistor to boost the current of LM741 op-amp.
Unidirectional means that the output current is available during the positive half cycle of the input signal. In the circuit diagram above, the LM741 op-amp is setup as non-inverting amplifier with the gain set up by the resistors R1 and R2.
\(Av = 1+ \frac{R1}{R2}\)
Here the voltage gain is 2.
R3 is the load resistor.
The maximum output current at the load is given by,
\(I_{max}=\beta_{dc}I{sc}\)
where \(I_{sc}\) is the short circuit current of the op-amp which is around 25mA.
The current gain \(\beta_{dc}\) of 2N3055 is typically 50 and has current rating of 15A and so the maximum current is,
\(I_{max}=\beta_{dc}I{sc} = 50 \times 25mA = 1.25A\)
Bidirectional Op-amp Current booster
The above implementation of current booster is unidirectional load current. Bidirectional means that the output voltage or current is available during both the negative and positive half cycle of the input signal. By using class-B push pull emitter follower we can implement bidirectional op-amp current booster. Such a circuit is shown below.
In the above bidirectional current booster circuit diagram, two complementary pair of transistor NPN 2N3055 and PNP MJE2955 are connected as class-B push pull amplifier at the output of the LM741 operational amplifier. When the input voltage is positive then the pnp transistor MJE2955 conducts and the output load voltage is negative. Similarly when the input voltage is negative going, the upper npn transistor 2N3055 conducts and the output load voltage is positive. And so in both the positive and negative going input signal voltage with high current appears at the load. The load current is higher because the output current of the op-amp is multiplied by either one of the transistor current gain beta.
Conclusion
Overall, power transistors whether it is BJT or FET transistor can be used at the output of op-amp to boost the output current on the load. Unidirectional and bidirectional current booster circuit were presented. In unidirectional current booster circuit, a single NPN bipolar transistor is used whereas in the bidirectional current booster circuit, two complementary transistors are used in class-B push pull configuration. Unidirectional means that current is conducting in the positive or negative goin cycle of the input signal. Bidirectional means that current is conducting during both the negative and positive cycle of the input signal.
