Sziklai Pair Amplifier

You need to supply a schematic diagram.

http://www.electro-tech-online.com/electronic-projects-design-ideas-reviews/102950-sziklai-pair-amplifier.html#post838791

the schematic diagram is attached

I would presume that the duty cycle of your 555 is wrong and it is producing the lower output voltage, try changing the % on / % off.

Disclaimer: I've never heard of a Sziklai pair before (had to look at Wikipedia), Electronic Wiz is far better qualified to comment.

You (Roronoa?) have implemented the Sziklai pair "up side down" but left the load at the top:

The schematic shown...................................................................Normal Sziklai pair

What technology is the 555: if it is powered by 12V can it drive close to the 12V rail?

I hope there is a series resistor between the 555 output and the input to the amplifier.

Depending on the technology of the 555: the existing circuit might work better if that pull down resistor is changed to a pull up resistor to ensure that Q3 turns off fully.

I used the free package TinyCAD to draw the above pictures. It helps a lot to understand what's going on if you don't try to work with circuit diagrams like this:-

Hello Chopper....

I didn't have time to comment more last night, I will now. Frankly, I hadn't heard of 'Sziklai' either, but no matter. It is simply a Darlington using complementary transistors. Randall is quite correct, you need to insert a resistor between the 555's pin 3 and the Darlington base, ~10K, since you don't need much drive current. In your schematic, the 2 Meg turns on the 2N3906 enough (it has fairly high gain itself) to cause some current flow even when you pulled the 555 out which accounted for your 8 volts. Randall is also correct about your Darlington, it is incorrectly connected. Also, while the output of the darlington will switch from near ground to near 12 volts, a meter will not read close to 12 volts unless the 555's duty cycle is off mostly (short on time). A scope will show the true output voltage. If your DVM has a true RMS AC function, for example, and the 555's duty cycle is 50%/50%, the meter will read about 6 volts. You will not get an accurate reading with an averaging AC meter nor a DC meter.

Chopper, you might give us a little more information on what you are doing with this circuit, what is the load it is driving besides the 270 ohm resistor? Does the output have to swing close to the supply rails or is close good enough?

Randall....the 555 originally came in bipolar transistor form, it has a totem-pole output, both active pull-up and pull-down (just like TTl logic), later CMOS versions were introduced which gave it a wider operating range of frequency but its output (pin 3) didn't have as much drive capability in sourcing (particularly) but could sink a fair amount. The CMOS 555's output swings closer to the supply rails than the bipolar 555 but in Chopper's instance, I doubt that would be of consequence since the 555 is only driving the base of the darlington to ground (actually, it should be called common not ground but who's counting). Both versions of the 555 are still in production by quite a few firms and both are cheap, it is one of the most popular ICs ever produced and is still a work horse of industry and hobbyists. There are a ton of apps out there for it.

the output swings anywhere near the supply rail will be good enough

The modifications we've mentioned will get you close to the 12 volt rail, be advised that your 'load' will draw its current through the 270 ohm resistor so that will be your primary voltage drop to your load when it is on. The Darlington should have only a few tenths of a volt drop across it when turned on 'hard', the 'load' is off. The Darlington should be quite 'off' when your load is on.

By the way, Chopper....

Your Darlington does work, it just isn't the 'Sziklai' Darlington you referred to. When the 555's output goes 'low' it will turn on the 2N3906 and 2N3904, when the 555's output goes 'high', normally this would essentially turn the 2N3906 off but the 2 Meg resistor will keep the 2N3906 turned on some. If you install the 10K in the 555's output as I said, you can eliminate the 2 Meg resistor, the 555's active output will pull the 2N3906's base up, effectively turning it off.

I gave Randall a GA for his good answer.

Chopper,

Have you looked at the data sheet of your transistors.

If you do a google search for 2N3906, one of the things which jumps out and hits you is the diagram of the pinout of the in line TO90 device

In your layout pin 3 of the 555 is clearly connected to the 2 Meg resistor and one of the end pins of the transistor in the 2N3906 position.

Of course you may be using transistors with a different pinout, but, you need to check the data sheets for both the transistors.

switch the input to the base of your Sziklai Pair.