Not sure what is going on here. Components test fine. Here is the circuit.

6V PSU. Input looks fine on the scope but only noise on the output. What have I missed here? PNPs are a bit new to my way of thinking so I've been working with them lately. But this one has me stumped.

 

Absents DC shift voltage on B-E Q1.

 

nput looks fine on the scope but only noise on the output. What have I missed here?

How do you expect to turn on Q1 with no DC base bias or path for DC (unidirectional) current?

 

Pretty obvious when I think about it. Was working from the diagram I was given... poor excuse I know... Thanks.

 

Even when its input is biased properly, it will have a voltage gain of only 1 (like an emitter follower).

 

it will have a voltage gain of only 1 (like an emitter follower).

Yes, it rather acts like a Darlington since it has a high gain, but has only one base-emitter drop instead of two.
Sometimes called a Complementary Darlington.

 

Well that just answered my second question as to why I was only getting unity gain. So, it is basically a buffer. Yes, I did try biasing Q1 with no effect. Hmmm... K, thanks guys!

 

It actually lost a few millivolts...

 

It actually lost a few millivolts...

Yes, an emitter follower gain is actually always slightly less than 1.

 

If the sine wave source is a function generator with an adjustable DC offset, set the offset to 3.6 V. The circuit output should be centered around 3.0 V.

What are you using for a scope?

ak

 

Siglent SDS2104 Plus and their SDG 1032X function generator. I normally use 10X for the probes but I get a better low voltage signal with 1X. Anything under ~500mV gets pretty fuzzy on the display for the input channel using a function generator input signal. Ouput signals fare much better.

set the offset to 3.6 V. The circuit output should be centered around 3.0 V.

Don't understand this. Please explain.

 

As others have pointed out, your circuit is missing a DC bias voltage at the base of Q1.. One way to do this is to add two resistors to the circuit at the base, one to Vcc and one to GND. These can be pretty high value parts, something in the 100K range, so they do not load the input signal appreciably.

In order to have the maximum possible output voltage swing range, you want the emitter to rest (no input signal) at Vcc/2. In your case that is 3 V, so the base should be held at 3.6 V. If you are not anticipating a large output signal, you can make the two added resistor equal valued. For example, if the two resistors are 100 K each, the base will sit at 3.0 V, the emitter will sit at 2.4 V, and, theoretically, the largest output voltage without clipping would be 4.8 Vpp.

BUT - all signals have both an AC value and a DC value. While we usually talk about a sine signal in terms of its amplitude (in volts RMS, peak, or peak-to-peak), there is an implicit understanding that the mathematical average of all of the instantaneous values of one full cycle of the waveform is 0 V. That is, the waveform spends as much time above GND as below GND. The positive and negative half-cycles are called that because the DC value of the waveform is 0 V, so the upper and lower portions of the wave are above and below GND; hence, positive and negative. In this case, the DC component of the waveform is 0 V. Your circuit requires that the DC value be greater than 0 V, so a DC offset must come from somewhere. It can be a two-resistor voltage divider, or . . .

If your function generator has the ability to add an adjustable DC offset to the sine wave output, then the function generator can supply the 3 Vdc by adding it to the sine wave as a constant offset voltage from GND. Now, as you turn the sine amplitude up and down, the DC voltage at the base will sti at 3 V. Note that for this to work, delete the 0.1 uF input coupling capacitor. Also, since your circuit is basically an emitter follower, you can delete the 1 K input resistor. The FG is supplying the DC bias voltage for the circuit to function correctly.

My cranky old FG is a Wavetek FG3B. The DC offset adjustment is the second knob from the right. It has a switch to enable/disable the offset. When disabled, the DC component of the output signal is 0.0 V. When enabled, it can be set to anything between +10 V and -10 V. For example, I could have a 0.1 V sinewave sitting a pedestal of 7.5 V.

Of course, there are limits. Don't expect the FG to be able to supply very much current. My FG can produce up to +/-20 V into a very high impedance, but only +/-10 V into 50 ohms. This implies that the maximum current from its output stage is +/-200 mA peak.

ak

 

Here is some of your signal voltage loss:

 

Actually, what I have done is to build this little bias module with a 100k pot and a 1k on each end for overrun safety.

Using the scope, I can fine tune the output using it by varying the input voltage and tweaking the pot to maximize the output so I can find the "correct values" pair of bias transistors for the particular transistor by then measuring each end of the pot's resistance value to the wiper with my ohm meter. So far, I've been pretty pleased with what it can do.

 

If your function generator has the ability to add an adjustable DC offset to the sine wave output, then the function generator can supply the 3 Vdc by adding it to the sine wave as a constant offset voltage from GND. Now, as you turn the sine amplitude up and down, the DC voltage at the base will sti at 3 V. Note that for this to work, delete the 0.1 uF input coupling capacitor. Also, since your circuit is basically an emitter follower, you can delete the 1 K input resistor. The FG is supplying the DC bias voltage for the circuit to function correctly.

Wow, this I did not know. Neat trick! There is very little information on using test instruments out there other than taking the basic measurements. Thanks! I have tried using it as a low voltage 60Hz AC source and found it pretty useless as it has almost no current. I'll have to do some experimenting with the DC offset. Yeah, DC offset won't work through the coupling capacitor. The 1k resistor was on the diagram I was working with but I eliminated it when I put my biasing module on the base although I did keep the coupling cap when I inputted the 1kHz signal from the sig gen. What I did notice was there was a very wide range of resistance values using my bias module that gave a perfect wave output. Which is very unusual as it typically is a very narrow range of tweaking the pot to get the correct output.

 

You do not need a pot to adjust the base voltage to be +3.6V. Instead you need two resistors as a voltage divider and some simple arithmetic since the base current of the first transistor is so low.

 

JFTR the emitter follower configuration won't give you any voltage gain, but it can provide plenty of current gain.

 

The equivalent current gain of a Darlington or a Sziklai Pair is basically the Beta of the first times the Beta of the second, so the input base bias current is very low.
The input bias resistors can thus be a relatively high resistance without significantly affecting the calculated bias value.

 

I did keep the coupling cap when I inputted the 1kHz signal from the sig gen.

Correct. If the DC bias voltage is supplied externally, you don't want the output stage of the signal generator to fight it with its own output DC level, which probably is 0 V.

If you put your scope on DC couple, then you should see the output sinewave stay the same amplitude while its DC pedestal changes with pot rotation.

ak

 

I have tried using it as a low voltage 60Hz AC source and found it pretty useless as it has almost no current.

There are tons of low-cost function generators on amazon and ebay. One of the things that sets them apart from more expensive "professional" models is the analog signal output amplifier. Mine can deliver +/-200 mA while maintaining a very clean triangle/sawtooth wave, especially around the zero-crossing, plus no little artifacts at the peaks.

ak