From our last article on this subject, we learned that a transistor can function exceptionally well in an amplifier circuit. Therefore, today we are going to learn how to put this knowledge to use by designing a small signal amplifier circuit using transistors.
A small signal source, such as a microphone or signal generator, often produces only a few tens of millivolts.
This level is far too low to drive a power amplifier directly, resulting in little or no audible output.
To solve this, we use a small signal amplifier (preamplifier) to raise the voltage to a usable level. A preamplifier is not about power, but about signal conditioning — preparing a weak signal for the next stage.
Designing a small signal amplifier circuit
In this article, we focus on the core design idea behind a small signal amplifier, keeping the explanation intuitive rather than calculation-heavy.
This approach is often sufficient for practical designs and helps build a solid understanding before moving to detailed analysis.
From my experience, using this method nets us below a 20% error rate in a calculation, which is acceptable in most electronic circuits.
In a typical audio application, a small signal amplifier is designed to increase a very low input voltage to a level suitable for a main amplifier stage.
At the same time, the circuit should operate with a low current, as most power amplifiers require only a minimal input drive.
These requirements can be easily met using a single small-signal transistor.
Choosing the transistor
For a small signal amplifier, most general-purpose NPN transistors are suitable, as long as they provide sufficient gain and can handle a modest collector current.
In practice, the exact transistor type is not critical. What matters more is stable operation and proper biasing.
In this example, a commonly available small-signal transistor is used for demonstration. A detailed comparison of suitable transistor types and selection guidelines is discussed separately.
Buy these small transistors at Amazon.com here (affiliated link)
Common emitter amplifier circuits
One of the most widely used high-gain transistor configurations is the common emitter amplifier.
In this arrangement, the input signal is applied at the base, the amplified signal is taken from the collector, and the emitter serves as the reference point.
This configuration offers high voltage gain and is therefore commonly used in preamplifier stages.
As we might have known, transistors require a bias to function. There are generally three ways to bias a transistor, depending on how we set up the base bias, which includes:
- Fixed Bias
- Self-feedback Bias
- Voltage-divider Bias
The practical differences between various biasing methods — and how they affect operating stability — are explained in detail in the extended design notes.
Making the circuit fall into class A
To achieve low distortion and predictable behavior, the amplifier is biased to operate in Class A mode.
In this mode, the transistor conducts continuously, which simplifies signal amplification at the cost of efficiency.
For small-signal stages, this trade-off is usually acceptable.
An emitter resistor is introduced to improve stability and limit excessive current.
In practical builds, the exact resistor values and bias currents matter more than the topology itself.
I documented the full design process — including calculations, measured voltages, and stability observations — separately for readers who want to build and tune this circuit in practice.
Determining the bias current and voltage
In transistor amplifiers, DC biasing establishes the operating point, while the AC signal is superimposed for amplification.
During analysis, the bias conditions are considered first, followed by the behavior of the AC signal through coupling capacitors.
This separation simplifies understanding and helps ensure stable operation.
About capacitors with AC signal
I am quite hesitant about telling my daughter the process of calculating the values of C1 and C2. Because it is quite complicated, as they operate in an AC, which has more variables than in a DC.
In this article, we use a try-out and experiment method to find the capacitors‘ values. But in the future, when we actually have fundamental knowledge, we may learn in-depth about this subject.
Like watering a plant, we should not overwater it when it is still a sapling, but we should start small and then gradually increase the amount of water as it grows.
Testing the circuit
After assembly, the circuit was tested to verify that the operating point remained stable.
When driven with a low-level audio signal, the amplifier produced clean and sufficient output for further signal processing stages.
This focuses on the design philosophy and structure of a self-biased small signal amplifier.
A complete, step-by-step bias analysis — including resistor calculations, operating point selection, and measured results — is documented separately for readers who want to build and fine-tune the circuit.
See an example circuit using this method: Simple Condenser Mic Preamplifier Circuit
Voltage-divider bias
Voltage-divider bias is an alternative biasing method that provides improved operating-point stability compared to simpler bias schemes.
By referencing the base voltage to a resistor network instead of relying on transistor gain alone, this approach reduces sensitivity to device variation and temperature changes.
A voltage-divider bias, or what may also be known as a bridge bias, gives out a very stable collector voltage with regard to any kind of transistor.
Testing this small signal amplifier circuit
In practice, the voltage-divider bias demonstrated noticeably improved stability compared to simpler biasing methods.
Once assembled, the circuit maintained a consistent operating point and delivered clean amplification under normal input conditions.
Conclusion
We saw that creating a small signal amplifier circuit using a transistor in a class A configuration is not too complicated.
We might be able to modify things, such as components, power supplies, etc, to get a different result from this circuit.
Small-signal transistor amplifiers can be biased in several ways, each with its own trade-offs between simplicity, stability, and flexibility.
Voltage-divider bias offers a reliable and predictable operating point, making it a common choice in practical designs.
Understanding why a particular bias method is chosen is often more valuable than memorizing calculation steps.
In the future, we might try to create a small amplifier circuit that uses three transistors instead of one. Be sure to let us know if this ideal interests you in some way.
Want the full design notes?
This article focuses on concepts and design choices.
The complete build notes — including detailed bias calculations, measured results, and design reasoning not shown on the website — are available for readers who prefer a deeper, practical reference.
If you find this work useful, you can explore them here.
Self-taught and driven by curiosity since childhood. I solve electronic challenges by getting my hands dirty at the workbench. My approach is simple: experiment, learn, and repeat.
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a materia merece um 10, altamente didatica, compreensiva, e muito gostosa de recordar, ja que tenho 3 faculdades de engenharia eletronica…mas esquecemos algumas teorias do passado.gostaria de receber terias sobre fontes chaveadas. formei a ultima em 1984 e essas fontes eram poucas exploradas, entao nao tive teoria aprofundada. no brasil nao se acha tecnico para conserto de fontes de pc justamente pela complexidade de funcionamento e peças dificeis de encontrar, parece que tem componentes que nao existem nos livros. diodos encapsulados..etc…obrigado por tudo
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