<div>### 60 V Single-Ended Class-A Transformer-Coupled Audio Amplifier
This circuit is the continuation of the earlier simple 9 V and 60 V common-emitter Class-A amplifier experiments.
The original circuit was first used to demonstrate transistor bias, common-emitter voltage gain, phase inversion, clipping, and the relationship between the quiescent operating point and available output swing.
The circuit was then scaled to a 60 V supply and finally converted into a transformer-coupled Class-A power-output stage capable of driving a low-impedance loudspeaker model.
### Main Components
#Vs provides the 60 V DC power supply.
#Qa is the NPN transistor operating as the single-ended Class-A output device.
#Rb supplies DC base bias and establishes the quiescent collector current of #Qa.
#ACs supplies the 1 kHz AC test signal.
#Cb is the 1 µF input coupling capacitor. It passes the AC signal from #ACs while preventing the signal generator from disturbing the transistor's DC bias.
#Vbe monitors the base-emitter voltage of #Qa.
#Vce monitors the collector-emitter voltage of #Qa.
The emitter of #Qa is connected to ground, so #Vbe is also the base voltage relative to ground.
### Class-A Bias
In Class-A operation, #Qa remains conducting during the complete signal cycle.
The transistor therefore has a substantial quiescent current even when #ACs has zero amplitude.
The input signal from #ACs is coupled through #Cb and produces small changes in #Vbe.
Those small base-emitter voltage changes produce much larger changes in collector current.
The relationship can be understood approximately as:
$$I_C \approx \beta I_B$$
where $\beta$ is the transistor's effective current gain.
During testing, the EveryCircuit transistor model behaved with an effective current gain close to:
$$\beta \approx 100$$
The base bias was therefore adjusted to obtain the desired standing collector current.
### Why the Transformer Was Added
The earlier version used a resistor as the collector load.
That arrangement successfully demonstrated voltage amplification, but a large collector resistor could not efficiently supply the high current required by a low-impedance loudspeaker.
The collector resistor was therefore replaced with an output transformer.
The transformer primary now carries the DC collector current of #Qa while also acting as the AC collector load.
The transformer transfers the AC signal to the secondary while preventing the collector DC voltage from being applied directly to the speaker.
This allows the transistor to operate from a high-voltage supply while delivering much larger current into a low-impedance output load.
### Transformer Operation
The transformer is configured as a center-tapped model in EveryCircuit.
The center tap is referenced to ground for simulator compatibility, while the loudspeaker load is connected across the secondary output.
The transformer is presently configured at approximately:
$$N_P:N_S = 17:8$$
For an ideal transformer, the voltage ratio follows:
$$\frac{V_P}{V_S}=\frac{N_P}{N_S}$$
and the impedance transformation follows:
$$\frac{Z_P}{Z_S}=
\left(\frac{N_P}{N_S}\right)^2$$
This means the low speaker impedance is reflected as a larger effective AC load at the collector.
The EveryCircuit center-tapped transformer model does not behave exactly like an ideal transformer, so measured voltages and currents were used when evaluating the actual simulated circuit.
### Transformer Losses
A 1 Ω resistor is placed in series with the primary to approximate primary winding resistance.
A 0.1 Ω resistor is placed in series with the secondary to approximate secondary winding resistance.
These resistors represent copper loss in a real transformer.
They must be placed in series with the windings rather than across them.
### Loudspeaker Model
The loudspeaker is represented electrically by #Rsp and #Lsp connected in series.
#Rsp = 6.5 Ω represents the DC resistance of the loudspeaker voice coil.
#Lsp = 500 µH represents the inductance of the voice coil.
#Vsp measures the AC voltage across the loudspeaker model.
A current probe is placed in series with the secondary load to measure actual loudspeaker current.
At 1 kHz, the reactance of #Lsp is:
$$X_L=2\pi fL$$
$$X_L=
2\pi(1000)(500\times10^{-6})$$
$$X_L\approx3.14\Omega$$
The approximate complex loudspeaker impedance is therefore:
$$Z_{sp}=6.5+j3.14\Omega$$
The magnitude is:
$$|Z_{sp}|=
\sqrt{6.5^2+3.14^2}$$
$$|Z_{sp}|\approx7.22\Omega$$
This is more realistic than representing an 8 Ω loudspeaker as a simple resistor because the inductive reactance increases with frequency.
### Common-Emitter Amplification
#Qa operates in common-emitter configuration.
When #Vbe rises, base current increases.
Collector current therefore increases.
Greater collector current produces a larger voltage change across the transformer primary, causing collector voltage to move downward.
When #Vbe falls, collector current decreases and the collector voltage rises.
The collector signal is therefore approximately 180° out of phase with the input signal.
### Collector Voltage Swing
Unlike the earlier resistor-loaded version, the collector does not need to sit near half the supply voltage.
Because the transformer primary has very little DC resistance, the quiescent collector voltage remains close to the 60 V supply.
During AC operation, energy stored in the transformer's magnetic field allows #Vce to swing substantially above the 60 V supply rail.
This is normal behavior for a transformer-coupled single-ended Class-A amplifier.
Measured examples showed #Vce swinging from only a few volts at the low end to well above 100 V at the high end before severe clipping occurred.
This also demonstrates why the transistor used in a real version must have a collector-emitter voltage rating substantially greater than the DC supply voltage.
### Clipping
The maximum clean input level was found experimentally by increasing the amplitude of #ACs while observing #Vce and #Vsp.
As the signal becomes too large, #Vce approaches the transistor's saturation voltage during one portion of the cycle.
When this happens, the collector waveform begins to flatten and distortion increases.
The useful maximum input is therefore slightly below the point where #Vce reaches saturation or the transistor reaches cutoff.
### Loaded Output Measurement
With the transformer configured at 17:8 and the complete loudspeaker circuit connected, one measured operating point produced approximately:
$$V_{sp(RMS)}\approx3.92V$$
and:
$$I_{sp(RMS)}\approx0.542A$$
The measured voltage and current agree with the approximate loudspeaker impedance:
$$\frac{3.92}{0.542}\approx7.23\Omega$$
which closely matches the calculated:
$$|Z_{sp}|\approx7.22\Omega$$
This confirms that the loudspeaker model is behaving consistently.
### Real Loudspeaker Power
Because #Lsp is reactive, not all apparent electrical power is dissipated as real power.
The real power is primarily dissipated in #Rsp.
Real loudspeaker power is therefore calculated from:
$$P_{sp}=I_{sp(RMS)}^2R_{sp}$$
Using the measured current:
$$P_{sp}=(0.542)^2(6.5)$$
$$P_{sp}\approx1.91W$$
The inductor stores and returns energy during each cycle but ideally does not dissipate average real power.
### 50 W Design Target
A later design goal was approximately 50 W of real power into the modeled loudspeaker.
For #Rsp = 6.5 Ω:
$$I_{RMS}=
\sqrt{\frac{P}{R}}$$
$$I_{RMS}=
\sqrt{\frac{50}{6.5}}$$
$$I_{RMS}\approx2.77A$$
The required peak current is:
$$I_{peak}\approx3.92A$$
Including the inductive reactance of #Lsp, approximately 20 V RMS across the full speaker model would be required at 1 kHz.
This demonstrates how quickly the current requirements increase when moving from a small-signal amplifier into a genuine power amplifier.
### Efficiency and Heat
Single-ended Class-A operation is inherently inefficient.
The transistor consumes substantial DC power even when no audio signal is present.
The approximate DC input power is:
$$P_{DC}=V_{CC}I_{CQ}$$
For example, a 60 V supply with approximately 1.8 A of quiescent current consumes:
$$P_{DC}=60(1.8)$$
$$P_{DC}\approx108W$$
even before useful output power is delivered.
Much of that power becomes heat in #Qa and losses in the transformer.
This is one of the defining characteristics of high-power Class-A operation.
### What This Circuit Demonstrates
This simulation demonstrates:
* Single-transistor Class-A amplification
* Common-emitter operation
* DC transistor bias
* Quiescent collector current
* AC input coupling
* 180° phase inversion
* Transformer-coupled output stages
* Transformer impedance transformation
* Collector voltage swing above the supply rail
* Transformer winding resistance
* Reactive loudspeaker loading
* Loudspeaker voltage and current measurement
* Real versus reactive power
* Clipping and transistor saturation
* The extreme heat and inefficiency of single-ended Class-A power amplification
### Purpose
This circuit is an educational simulation intended to explore the electrical behavior of a single-ended transformer-coupled Class-A audio amplifier.
It is not intended as a construction-ready high-power amplifier.
A real implementation would require appropriate analysis of transistor safe operating area, collector-emitter voltage rating, collector current rating, power dissipation, heat sinking, transformer core saturation, transformer air gap, winding current capacity, speaker impedance versus frequency, power-supply capability, thermal stability, fusing, and protection circuitry.
The next stage of experimentation is to investigate active current regulation and thermal stabilization, including the possibility of using a constant-current circuit to maintain a stable operating point when resistive elements change value with temperature.
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