# How to design discrete circuits for transistor beta changes?

• dpsciarrino
In summary, negative feedback is used to compensate for fluctuations in the beta parameter of discrete transistor components, which can be avoided by using circuits that are tolerant of beta variations.
dpsciarrino
In my circuits/active devices course we had to make amplifier designs with various type of transistors (BJT, JFET, MOSFET). In each design the transistor beta parameter played an important role in deciding which resistors to use in the final amplifier circuit. This beta parameter changes widely from one transistor to another...

My question is, if I wanted to manufacture a circuit that has discrete transistor components, how are these fluctuations in the beta parameter of all these transistors compensated?

For example: If two 2N3904s are used and their betas were, say, 100 and 130, would the resistors and caps in the design have to cater to each individual transistor, therefore CHANGING the design values of the circuit for each individual product? Or are there some useful circuits that get around this pitfall? Or are there other techniques that I don't see?

Pretty much the first topic covered in BJT amplifiers in school was designing in such a way as to minimize the effect of changing beta.

Check out "The Art of Electronics" by Horowitz and Hill. They've just issued a new edition. It's full of helpful and practical approaches to problems like this. It starts right at the beginner level but works right through things like transistor & FET biasing and reducing beta and gm sensitivity. They cover a multitude of topics and have all kinds of real-world examples, part numbers, and practical tips. The tone is informal and entertaining too.

berkeman

## 1. What is transistor beta and why does it change?

Transistor beta, also known as the current gain, is a measure of the amplification capability of a transistor. It is the ratio of the output current to the input current in a transistor. The value of beta can change due to variations in temperature, operating conditions, and differences in manufacturing processes.

## 2. How does transistor beta affect circuit design?

The value of transistor beta is an important parameter to consider in circuit design. It determines the amount of current amplification and can affect the overall performance and stability of the circuit. A change in beta can lead to changes in the operating point, gain, and frequency response of the circuit.

## 3. What factors can cause transistor beta to change?

Transistor beta can change due to variations in temperature, which can affect the mobility of the charge carriers. It can also change with changes in the operating voltage and current, as well as manufacturing process variations.

## 4. How can I design a circuit to compensate for changes in transistor beta?

There are several techniques that can be used to compensate for changes in transistor beta. One approach is to use negative feedback to stabilize the circuit and reduce the impact of beta changes. Another method is to use a biasing circuit that can automatically adjust the operating point based on the actual value of beta.

## 5. Are there any tools or software available to help with designing circuits for transistor beta changes?

Yes, there are various circuit simulation software programs available that can help with designing circuits for transistor beta changes. These programs allow you to simulate different scenarios and analyze the impact of beta variations on circuit performance. Additionally, there are also online calculators and tools that can help with calculating and selecting components for a desired beta value in a circuit design.

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