Transformer Coupled Class A amplifier Single supply

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Summary:

How can Vce vary from 0 to 2Vcc? In class, I learned that it was due to Lenz Law working in the transformer and a voltage of Vcc being induced to reverse the change in Ic. However, I cannot picture this. Can someone please explain with a diagram of how this comes about in the transformer windings.
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How can Vce vary from 0 to 2Vcc? In class, I learned that it was due to Lenz Law working in the transformer and a voltage of Vcc being induced to reverse the change in Ic. However, I cannot picture this. Can someone please explain with a diagram of how this comes about in the transformer windings.
 

Answers and Replies

  • #2
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How can Vce vary from 0 to 2Vcc? In class, I learned that it was due to Lenz Law working in the transformer and a voltage of Vcc being induced to reverse the change in Ic
No need to think about Lenz's law, only need to consider the superposition of DC bias point and AC signal fluctuations.

However, I cannot picture this. Can someone please explain with a diagram of how this comes about in the transformer windings
I think the diagram you posted is enough to describe the situation.

For the AC signal circuit, the effective load resistance of V1 is equal to (N1/N2)2RL=Re

In AC circuits, due to the constant effective load resistance Re, also the symmetry of positive and negative current fluctuations, the amplitude of positive voltage fluctuation will be same as the amplitude of negative voltage fluctuation.

Since negative Voltage fluctuation is Vcc, the positive Voltage fluctuation will be also Vcc

Therefore, the positive peak voltage on the collector of the transistor is equal to the sum of DC bias and positive voltage fluctuation, namely Vcc + Vcc = 2Vcc
 
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  • #3
DaveE
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For an ideal transformer (which doesn't exist, of course), you will only get Vcc as your intuition suggests. Then the circuit is equivalent to a simple load resistor in place of the transformer (Re as described above).

In the real world, the transformer primary will also have some inductance. This can generate an induced voltage to "try" and maintain current flow through the primary when the transistor turns off. In the extreme case, you could replace the transformer with an inductor. In fact it is easy to generate much more than 2Vcc this way. For example this is how many cars make the HV pulse for their spark plugs. To learn more about this effect you could search for "Flyback Power Supply" or "Boost Converter". However, these circuits aren't operated in the linear region like a Class A Amp, in those examples the transistor is switched on/off quickly.

In practice, if I saw this schematic, my response would be "tell me more about the transformer" and maybe "what is this circuit supposed to do?"

Actually modeling a real transformer is a bit complicated, ask if you want more info.
 
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  • #4
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In order to better explain the working principle of the circuit, I agree that the magnetizing inductance of the transformer must be retained in the circuit diagram. In addition, the value of the magnetizing inductance cannot be infinite, and actually must be appropriately selected as a certain value.

If we do not include the magnetizing inductance, or assume that the magnetizing inductance is infinitive large, then the "ideally transformed" load would be only a pure resistance Re. We would mistakenly conclude that the quiescent collector voltage must be ## ~\frac {Vcc} 2 ~## rather than ## ~Vcc ~##.

123.jpg


After adding the magnetizing inductance, we can also notice that the frequency response does not extend all the way down to DC, because at low frequencies, it will be shorted out by the magnetizing inductance.
 
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