Utilise audio amps for voltage splitting (Part 3)
Voltage splitters based on power audio amplifiers
From the point of view of the power supply the audio amplifiers are generally of two types. The first type is used mainly in applications with a single power supply. Some can be used with a dual power supply but that is an unusual application and sometimes it is not possible. Examples of these types of PAAs are LM386, LM380, LM384, TBA820M and similar.
The second type of PAAs are used mainly in applications with a dual power supply, but some simple additional circuit can work also with a single power supply. Examples of these types of PAAs are TDA2030, TDA2040, TDA2050, LM1875 and similar ICs. Although there are a lot of similarities between these PAAs we should note a lot of differences and particularities between them.
Figure 6 presents the circuit diagram of a very simple non-adjustable voltage splitter using an LM386.
Figure 6: Non adjustable voltage or adjustable splitter with LM386.
The circuit works in the full power supply range of the IC, but the output current is limited by the power dissipation of the IC, which depends on the power supply. Resistors R2 and R3 are not obligatory but have a protection role (e.g., if we omit them we should take special consideration of the pcb layout because the inputs of LM386 can capture a lot of noise and that noise will be amplified and will be seen at the output of the IC).
The input resistance of the LM386 is around 50k and the inputs are with PNP transistors and can go to ground. Resistor R4 may be necessary for the stability of the IC.
The trimmer potentiometer P1 is used to adjust the output voltage. We can apply the offset from P1 to the inverting or non-inverting input of the IC. Jumper J1 is for application of the offset to the non-inverting input and J2 is for the application of the offset to the inverting input. J1, J2, R1 and P1 are not obligatory and can be omitted in the simplest case.
Sometimes we are in need of more current, higher voltages and higher power dissipation than those offered by the LM386. Also we may need to adjust the output voltages from the VS. In some of these cases we may use more powerful ICs such as the LM380 and LM384. These two ICs have practically the same internal circuit, but the LM384 can provide more power. Also, the LM380 is produced in 8- and 14-pin packages while the LM384 is produced only in a 14-pin package. Both ICs can require additional heat sinks.
Figure 7 presents the circuit which can be used as an adjustable or non-adjustable VS.
Figure 7: Adjustable or non-adjustable voltage splitter with LM380 in a DIP8 and with LM380 or LM384 in a 14-pin DIP.
The first half of the circuit is a VS with a LM380 in an 8-pin package; the second half of the circuit is a VS with an LM380 or LM384 in a 14-pin package. Both parts of the circuit are independent.
The jumpers J1 to J4 can be used for adjustment of the output voltages. In all cases the input voltage of IC1 will be amplified around 50 times and will appear between the output of the IC and GND1. That gain should be taken into consideration if we wish to adjust the output voltage with a trimmer potentiometer.
In some cases we may wish to have a stable output voltage between the output of the VS and the input ground terminal. In these cases we should apply a reference voltage at the appropriate input of the PAA.
Figure 8 presents simple adjustable voltage splitter with an LM386.
Figure 8: An adjustable voltage splitter with an LM386 and with a reference input voltage from an LM385-1.2.