Transformer Coupled Class A Amplifier - Power Stage Question

[Gaussian]

I'm new to building audio amplifiers. I have successfully tested a variety of amplifier circuits using RC coupling, so I decided to test a circuit that uses transformer coupling.

I built a two stage amplifier using a CE connected PN2222A transistor in the voltage stage (I_C=1mA, V_CE=10V) and a CE connected TIP50 in the power stage (I_C=300mA, V_CE=10V). I have a Hammond 109F AF transformer connected to collector on the TIP50 (2:1 turns ratio). I'm using a value of r_o=33.33 ohms for the TIP50's output resistance (V_CE/I_C). I have 33 ohms of resistance on R_E with ~2411 uF of capacitance in parallel. My load is an 8 ohm speaker.

Using R'_L = (N_P/N_S)²R_L with R'_L=33.33 ohms and R_L=8 ohms I should be matching impedance between the TIP50's output and the load resistance using the 2:1 Hammond 109F AF transformer.

The amplifier actually performs quite well. There's very little distortion, and the frequency response is pretty good (better than I expected).

My question has to do with the values of I_C and V_CE I'm measuring on the power transistor. With no ac signal applied, I expected to see a collector current around 300mA and a collector-emitter voltage around 10V for a properly biased circuit operating in the active region. Instead, I'm seeing a collector current around 56mA and a collector-emitter voltage around 16V. Is this expected, and if so, why?

When I build the same circuit using capacitive coupling on the output, I get the expected results (300mA and 10V).

Thanks, in advance, for your help.

[pardo-bsso]

How do you arrive at the conclusion that Q2 should bias around 300mA and 10V?

When you say "capacitive coupling", where do you put that capacitor?

[Gaussian]

From the data sheet for the TIP50 which can be found here:

https://www.st.com/resource/en/datasheet/tip50.pdf

Under DC current gain, (h_FE has a min/max of 30 and 150 respectively (I used 67, ((30)(150))^1/2=67) for I_C=300mA and V_CE=10V. I have biased and stabilized the TIP50 this way. Thus, the quiescent current (I_CQ=300mA and the quiescent voltage (V_CEQ=10V) puts the Q-point in the middle of the DC load line. The transistor should be operating in the middle of its active region allowing for maximum swing (no clipping).

When I tested the circuit using capacitive coupling, I modified the circuit such that R_C2=27 ohms and R_E2= 6 ohms (I also keep C_E2 - albeit a different value - in place). The output is then connected between R_C2 and the transistor (just like Q1 is connected). The coupling capacitor on the output is in series with the load, R_L. C_C2 is calculated based on R_L||R_AC.

[duak]

Gaussian, do you have the specs on the output transformer?  Do you know what the maximum primary current is?  Do you know if it is designed for single-ended or push-pull operation?  If it's not designed for single-ended operation it can be saturated by the output stage quiescent current.

With no input signal, the DC voltage across the transformer's primary should be quite low making Q2's collector voltage approach Vcc.  The transformer's primary AC impedance will be much higher than its DC resistance so with an input signal you'll see a large AC signal.

As a rule of thumb for a circuit like this, Q2's emitter voltage is usually around 0.6 V to Vcc /10 to allow for maximum collector voltage swing.  The voltage divider formed by R3 & R4 sets the base to 11.2 V.  The emitter will be about 10.6 V and should cause the emitter current to be about 321 mA.  I would both change the ratio of R4 to R3 to set the base voltage to about 1.6 V and also increase their values into the K ohm range.  Please note that for the AC signal, R3 is in parallel with R4 and presents a combined impedance of 117 ohms to the first stage.  The output impedance of the first stage is essentially RC1 in parallel with the output admittance of Q1 and will be something on the order of 8K ohms and will be seriously loaded down by the output stage, reducing its effective voltage gain by 20X or so.

In the early days, transistor circuits used configurations based on those used with vacuum tubes.  Tubes have higher input impedance and so there is less of a problem with interstage loading.  With transistors, DC coupling solves much of this as well as simplifies the circuits.  There are more considerations but I'll stop now to let you digest the above.  Hope it makes sense.

Cheers,

[Circlotron]

With an ideal transformer, under no signal conditions the collector voltage would be equal to Vcc. At full output the collector voltage would swing from zero volts to twice Vcc. A transformer used this way needs to have a gapped core so that it will tolerate dc current.

[Gaussian]

Thank you very much duak - that's really helpful information. I was unaware of the potential for core saturation problems using a ferromagnetic core. I haven't seen this mentioned in any of the materials I've researched so far. I will revisit my circuit and try the things you've mentioned. I truly appreciate your help.

The spec sheet for the Hammond 109F is here:

http://www.hammondmfg.com/pdf/109F.pdf

You are right, it's not the right type of transformer for this application. I can probably use it in a push-pull amp though 

Can you, by any chance, recommend any good books on amplifier design that offer this level of detail? I have been referencing "Principles of Electronics" by V.K. Mehta which has been great, but it doesn't offer much detail when you get to transformer coupling. I have been thinking about buying "Designing Audio Power Amplifiers" by Bob Cordell.

Again, thanks much!

[Gaussian]

Thank you Circlotron. I actually stumbled across this video yesterday that expands upon the design principles you shared:



I will be taking the advice from you and duak to obtain an air core transformer for use in this circuit.

Thank you for your help!

[duak]

Gaussian, I still have some books on transistor amplifier design from high school back in the 70's.  Both of these books were well into DC coupled circuits without transformers but they do show some with.  If you want material with more detail I think you'll have to look for books, papers or app notes from the 50's and 60's.  I remember both the high school library and the city library having these kinds of books but just 20 years later they were pretty much gone.  If you are close to a university or technical college library, they might have something to at least have a look at.

 Handbook of Modern Solid-state Amplifiers, 1974 edition, John D Lenk -- has circuits, considerations and calculations for amplifier design.

 RCA Solid-State Devices Manual SC-16 -- circuits and considerations mostly.  More techno sales blurb than engineering.  RCA had a whole series of handbooks for vacuum tubes and solid state devices showing typical applications and design principles.

Do you have a particular goal in mind?  Since you have a push-pull transformer, why not try to use it?  If you're after a class-A output stage, you can build a push-pull version as the DC quiescent currents of the two transistors will cancel and the transformer will not saturate.

Cheers,

[Gaussian]

Thanks again duak. I really appreciate your help.

I don't have any particular goals in mind. I've just wanted to experiment with amplifiers so I thought I'd start with a class A amplifier and go on from there.

That said, air core audio transformers are a little hard to find (and a bit expensive when you do find them). I think I'm going to move on to push-pull amplifiers as you suggested.

While this circuit has some issues, the transformer coupling is working to some reasonable extent. I'm able to drive an 8 ohm speaker that I commandeered from an old guitar amp and it actually sounds pretty good - even with the core saturation. I'm sure the true audiophiles here would disagree. 

I may visit one of the local university libraries as you suggested. That's another good idea.