Two-Transistor Common-Emitter Amplifier with Current-Boost Output

The common-emitter amplifier (CE) is widely used due to its simplicity and high voltage gain. However, while it can easily increase signal amplitude, its output impedance is relatively high, limiting its ability to drive lower-impedance loads without distortion or signal loss. In practical applications, this is frequently a noticeable limitation.

In this article, we’ll build and test a two-transistor amplifier that has a common-emitter voltage gain stage and a common-collector (emitter follower) output stage. The common-emitter section increases signal amplitude, whereas the emitter follower improves current drive and reduces output impedance. The end result is a small and useful circuit that strengthens weak signals while maintaining waveform integrity.

The Goals Of this Circuit

Imagine that you just finished a fantastic sine-wave oscillator circuit. However, the output voltage is quite low, definitely not suitable for driving a load (like our basic LC oscillator circuit). What can you do here?

One simple solution is to use a simple, single-stage amplifier. This small circuit will increase, or amplify, the oscillator voltage potential and drive capability. Before that, let’s set a simple goal for this amplifier circuit.

• It should have, at the very least, approximately 20 voltage gain. (e.g., 0.6Vp-p input to 10Vp-p output).
• It should be able to drive a reasonably sized load relative to the transistor used.
• It had to be comprised mainly of transistors and a few other basic components.
• It must not have any negative effect on the output waveform.

As you can tell, the goal of this circuit isn’t a high-voltage-gain amplifier. Instead, we want a simple one we can put together quickly with reasonable voltage gain and low-impedance output.

For this task, we planned to use a single-stage common-emitter (CE) amplifier circuit. It’s one of the most basic transistor amplifier circuits, and we have used it many times before. But first, let’s do a quick brush-up on what common-emitter amplifiers are all about.

Short Recap On The Common-Emitter Amplifier

A common-emitter (CE) amplifier is one of the basic, single-stage BJT amplifier circuits alongside common-base (CB) and common-collector (CC). A CE circuit is known for its high voltage gain and decent current gain. 

In its simplest form, a CE circuit consists of an NPN transistor with its Base connected to the amplifier input. Its Emitter to the ground (GND). Its Collector to Vcc via a collector resistor (Rc) while also acting as the amplifier output. There would also be a couple of resistors working as bias components (omitted in the simplified schematic above).

Another attribute unique to a CE amplifier is that the output is inverted compared to the input. For instance, if we apply a sine-wave input, the output is out of phase by 180°. That said, for a continuous sine wave, signal inversion shouldn’t have any real impact on the load.

However, like other BJT amplifiers (CC and CB), the CE amplifier doesn’t invert the output current. In other words, both the input and output currents are in phase.

How It Works

In an ideal CE amplifier (which we assume this simplified version is), the transistor and the collector resistor function together as a voltage divider. Where the resistance of Rc is fixed, but the resistance between C and E is determined by the input base current (Ib). Put simply, the transistor is a current-controlled potentiometer. 

For example, if the Vbe and Ib increase, the C-E resistance decreases, causing more Ic current to flow through the transistor. And if you recall, when the resistance of the second resistor in a voltage divider drops, the output voltage will also drop. As a result, when the input voltage (Vbe) rises, the output voltage (Vce) drops.

On the other hand, if the Vbe and Ib decrease, the C-E resistance increases, reducing the Ic current. We can say that when the input voltage (Vbe) drops, the output voltage (Vce) rises.

These relations can be summed up into the following two cause-and-effect orders.

• Vbe rises → Ib rises → Ic rises → Vce drops.
• Vbe drops → Ib drops → Ic drops → Vce rises.

We can see that the output current (Ic) changes in the same direction as the input (Ib and Vbe), whereas the output voltage (Vce) is opposite, inverted compared to the input. 

In practice, the C-E resistance of the transistor changes only when the base voltage (Vbe) is greater than 0.6V. Furthermore, the Ib current wouldn’t create an equal amount of Ic. The Ic is always equal to Ib multiplied by the transistor gain (sometimes β or hFE), which is where the amplifying effect comes from.

We can easily work out the Vce, or the output voltage. If we know the input current (Ib), we can find Ic by multiplying Ib by the transistor gain. Since Ic must be the same as the current over RC (Irc), we can find Vrc using Ohm’s law, Vrc = Ic × Rc.

Now, since this circuit is effectively a voltage divider, we can be sure that the Vrc and Vce voltages must add up to Vcc, which means Vce = Vcc – Vrc. If we combine these steps together, we’ll get Vce = Vcc – (Ib × β) × Rc.

Theoretical Circuit

To see this process in action, let’s think up a hypothetical CE amplifier circuit. What we know about this circuit includes a 10V supply voltage (Vcc), an Rc resistance of 2.5kΩ, a transistor gain (β) of 100, and an input impedance (Rin) of 100kΩ.

The input signal is a sine wave at an undetermined frequency. The signal high-peak voltage is 3V, and the low-peak is 1V, or 2Vp-p. What can we do here?

First, since we know the transistor input impedance (Rin), we can determine the base current (Ib). Based on the input voltage swing from 1V to 3V and a 100kΩ Rin, we can see that Ib varies from 10μA to 30μA. The average of Ib is the Q-point, which is 20μA; Ib swings up and down by 10μA relative to the Q-point.

Next, we can work out the output or collector current (Ic) by multiplying Ib by the transistor gain (β) of 100. This shows Ic swings from 1mA at the low peak to 3mA at the high peak, which is 100 times Ib at any given point (Ic = Ib × β).

Now that we have Ic, we can use it to approximate Vce. First, we find Vrc by multiplying Ic by the known value of Rc, 2.5kΩ. Then, we know that Vce is always the difference between Vcc and Vrc, so we basically flip Vrc over to get Vce (Vce = Vcc – Vrc).

The result shows Vce swings from 2.5V to 7.5V, or a 2.5V swing up and down from the 5V operating point. In other words, the output voltage has a peak-to-peak difference of 5Vp-p. Since the input signal is 2Vp-p and the output 5Vp-p, we can say that the voltage gain of this hypothetical CE amplifier is 2.5.

The graph also clearly shows that the output current Ic is in phase with the inputs, while the output voltage is not. The output being inverted from the input, or 180° out of phase, is one of the identifying features of a CE amplifier.

With that said, this is just a simplified, theoretical circuit. There is a very high chance it wouldn’t work at all in reality. For instance, 100kΩ for transistor input impedance is quite far-fetched; most had less than 10kΩ, which, in this setup, wouldn’t satisfy the 0.6V cutoff or turn-on point.

The real circuit has many other components or factors to adjust and fine-tune the amplifier. However, knowing these basic theories will help us greatly when creating a real, usable circuit. Speaking of which, let’s move on over to building real, usable common-emitter amplifier circuits.

CE Amplifier Circuit With Common-Collector Current Booster

Common-collector—or emitter follower—is another type of BJT amplifier setup. Unlike the CE amplifier in the first stage, the CC amplifier provides no voltage gain. Instead, it significantly boosts the output current, which is why it’s mostly used as a last-stage current booster.

The circuit is quite simple, consisting of two transistors (Q1 for the first CE stage and Q2 for the CC stage), a half dozen bias and loading resistors, and a potentiometer for amplitude adjustment.

This circuit, however, does not use an output AC-coupling capacitor, so the common-collector stage output sits at approximately half the supply voltage (about 5V in our measurements). Instead of being centered at 0V and swinging between +5V and −5V, the signal is centered around 5V and swings upward toward 10V and downward toward 0V.

If we want both high-current drive and a ground-centered (0V) output with true positive and negative swings, we would typically need a dual-rail (±) power supply. Such circuits require additional biasing considerations and more components, which are beyond the scope of today’s article. Maybe in the future, we’ll take a closer look at them.

Output Impedance Test

To observe the increased output-driving capability, we place a 1kΩ load on the output. At 10V output, a 1kΩ load should, in theory, pull approximately 10mA of current. However, because we lack an AC ammeter, we cannot directly measure the load current.

The oscilloscope reading shows no decrease in output amplitude even with the 1kΩ load in place. The voltage is the same as without the CC stage, 10Vp-p. The waveform also remains affected.

The input signal we used is a 34kHz sine-wave signal, which has a voltage of 0.6Vp-p. Increasing that to 10Vp-p means that this circuit has about 17× voltage gain.

Notice R4 (Re) in the real circuit; we used a timer instead of a normal resistor. It served two purposes: First, we simply don’t have a low-wattage 47Ω resistor. Secondly, adjusting the emitter-degradation resistor is a quick and crude way to change the stage gain. And during our testing, we lowered R4, and the output went closer to 12Vp-p with the same input, albeit with slight distortions from the lack of voltage headroom.

Conclusion

All in all, the common-emitter amplifier with the common-collector current booster performs as intended and meets the design goals outlined at the beginning. It is capable of amplifying a 0.6Vp-p input signal to approximately 10Vp-p and driving a 1kΩ load without noticeable waveform distortion. The circuit also remains simple, using only two BC549B transistors and a small number of passive components.

That said, it is not without limitations. The output is DC-biased at approximately half the supply voltage due to the single-supply design, and the overall voltage gain is relatively modest (around 20×). However, considering its simplicity and improved current-drive capability, the circuit offers a practical and effective solution for boosting low-level signals.

In a future article, we may explore alternative amplifier designs with different biasing methods and characteristics.

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