Output transistors
This LM1875 based simple amplifier can work with wide variety of output transistors. The output current and output power of the amplifier depends on the selected transistors and the cooling devices.
It is preferably to use transistors with
* maximal continuous collector current above 6A,
* maximal collector-emitter voltage above 60V,
* maximal power dissipation at +25C above 65W,
* transit frequency above 2.5MHz,
* h21e above 20 at large collector current (e.g. above 5A) and
* maximal working temperature more than 150C (preferably 200C).
Appropriate couples of transistors are:
BD907 (NPN) + BD908 (PNP),
BD909 (NPN) + BD910 (PNP),
BD911 (NPN) + BD912 (PNP),
2N3055 (NPN)+ MJE2955 (PNP),
TIP3055 (NPN)+ TIP2955 (PNP),
MJE3055 (NPN) + MJE2955 (PNP)
2SC5200 (NPN) + 2SC1943(PNP),
MJL4281A(NPN) + MJL4302A(PNP)
2N3773 (NPN) + 2N6609 (PNP)
BD249 (NPN) + BD250 (PNP),
TIP33C(NPN) + TIP34C(PNP)
TIP35C(NPN)+TIP36C(PNP),
TIP41C (NPN)+ TIP42C (PNP),
and similar or better.
AC and DC operations
The amplifier from Figure 1 can work as AC and DC amplifier. When switches S1 and S2 are closed the amplified is working as DC amplifier. When switches S1 and S2 are open the amplifier is working as DC amplifier.
Heat sink
The amplifier should work in all cases with appropriate heat sink, e.g. with thermal resistance below 0.7C/W. It is preferably to apply small fan (e.g. 12V/0.1A – 0.3A) over the heat sink in order to improve the cooling. The fan should be activated when the temperature of the heat sink is higher than 50-60C.
Power supply of the amplifier
The amplifier can work with power supply in the full power supply range of LM1875.
The power supply can be regulated or not regulated and should be selected according to the configuration of the amplifier for a particular application.
Construction & testing
For testing and experimenting purposes we in need of power AC or DC amplifier with low cost and easily reparable. In these cases the THD and the IMD of the amplifier are not very important. It is important to have simple solution of power amplifier which can be adapted to a particular application with external passive filters at the input and the output.
Also it is interesting to use the same amplifier with different configurations of elements (kind of modularity or configurability) as output transistors, filtering capacitors, type of the heat sinks, etc. to adapt the implementation to different field of application.
Advantages of the amplifier
Although there are ICs providing similar and sometimes higher output power this LM1875 based simple amplifier with output transistors from Figure 1 has several advantages:
* The total maximal power dissipation of the transistors is higher than the power dissipation of the IC providing similar output power;
* If damaged one or more output transistors can be replaced easily because they have only three pins.
* The maximal output current (both DC and AC) of the amplifier with transistors can be significant, e.g. more than 15A.
* The maximal working temperature of the transistors can reach 200C (for most of the ICs it is around 150C),
* The amplifier can work with many popular and low cost transistors,
etc.
Feel interested? Check out other electronics projects.
Polarity of the capacitor C6,C8,C10,C12,C14 are wrong. In this circuit positive terminal of the capacitors are connected to the ground (-ve) pin.
Need to add Crossover distortion network in before transistor stage. This circuit won’t amplify signals between +0.7v to – 0.7v which is fed to base of transistor.
İn audio power amplifiers,the output stage have to work at AB class.There is no bias cırcuıt ta achieve AB class.With these condition this can only work for power inverter.
LM1875 delivers POUT=20W, RL=4Ω, fo=1 kHz with 0.022% THD.
What is the additional distortions contributed by the external PNP and NPN transistors?