# Having trouble understanding transistor data sheets and characteristic curves

#### FuneralHomeJanitor

Joined Oct 12, 2019
42
I have been trying to design and later assemble a simple class A common emitter amplifier myself and have been trying to learn on my own how to do this but am stuck and have been for a while because I don't understand how to read and interpret the datasheet(ON Characteristics) in order to find the proper values I need to design this circuit. I understand the circuit and how it works and what the different values in the circuit mean, but I would like to build a circuit from the start and calculate any values I need based on the datasheet. The problem is any example I see has values given already(such as load resistor value or collector current value) and I don't know how I would figure this out using a datasheet. I am confused by the 5 values given for the DC current gain on the datasheet and as a result cannot figure out how to find the resistor values I would need. I also understand the characteristic curve to some extent but don't know where it comes from. Is it something I draw using the DC current gain values? Is this why there are different values given? Is it something given by the manufacturer? Do I choose a resistor value to use to calculate collector current or do I use collector current from the datasheet to find my resistor value? It's like there are one too many unknowns for me to create a simple class A amplifier and I don't know where to actually start in order to start making the calculations. I know once I have one value I will easily be able to make more calculations but don't know what this value is or how to find it. For reference I am attempting to use a 5v supply and a 2n2222 transistor but I eventually want to be able to create this circuit with different sources and transistors. Thanks in advance and hopefully this isn't a dumb question.

#### crutschow

Joined Mar 14, 2008
33,355
You are working in the wrong direction.
You can't directly use the transistor data sheet to determine the design values for your circuit.
You determine what the circuit needs to do (power supply voltage, desired gain, frequency response, load to be driven. circuit current, etc.), and then you pick a transistor that will work in your circuit based upon its current and voltage limits.
Then you use the data sheet to do the design.

I am confused by the 5 values given for the DC current gain
Best design practice is use the lowest value at the current your circuit will be operating at.
I also understand the characteristic curve to some extent but don't know where it comes from.
Typically it consists of stepping the input (base-emitter) current in small increments, and then sweeping the collector voltage from zero to some maximum below the transistor rating, which then gives collector current versus voltage for each value of input current.
It's rather a snapshot of the transistors typical operating characteristics.

This brings up an important point.
The best design practice is to use worst-case, not typical transistor parameters in your design.

#### Audioguru again

Joined Oct 21, 2019
6,441
The current gain depends on the current as shown on the 2N2222 datasheet with 7 different amounts of current. The range of current gain is shown as 100 to 300 only when the current is 150mA which is a high current. A 2N3904 transistor has its range of current gain shown at 10mA.
The curves in the datasheet show the effect of current and temperature affecting the Vbe.
Then an emitter to ground resistor is needed for reducing these effects by applying some negative DC feedback.

#### dl324

Joined Mar 30, 2015
16,146
Welcome to AAC!
It's like there are one too many unknowns for me to create a simple class A amplifier and I don't know where to actually start in order to start making the calculations.

First you need to know the amplitude of the input signal and how much it can be loaded. Then you need to determine what you're going to be driving.

Once you know the voltages and currents required, you can start looking for an appropriate transistor.

If I understand what you mean by common emitter amplifier (you should provide a schematic so we don't have to guess) you don't want to use a grounded emitter because you'll likely saturate the transistor.

#### MrChips

Joined Oct 2, 2009
29,848
This is your basic common emitter amplifier circuit.

You begin by specifying the input voltage and current, and the output voltage and current.
In other words, you need to have an idea of the input impedance, the output impedance, and the voltage gain.

You have begun at the right place by specifying +Vcc = 5V.
Now, decide the desired output current. For example, 1mA at 4V will set RL to approx. 4kΩ.

Next, determine how much voltage gain is desired. This will determine RE.

Voltage gain ≅ RL / RE

The input impedance is going to determine the values of base bias resistors R1 and R2.

#### BobaMosfet

Joined Jul 1, 2009
2,102
I have been trying to design and later assemble a simple class A common emitter amplifier myself and have been trying to learn on my own how to do this but am stuck and have been for a while because I don't understand how to read and interpret the datasheet(ON Characteristics) in order to find the proper values I need to design this circuit. I understand the circuit and how it works and what the different values in the circuit mean, but I would like to build a circuit from the start and calculate any values I need based on the datasheet. The problem is any example I see has values given already(such as load resistor value or collector current value) and I don't know how I would figure this out using a datasheet. I am confused by the 5 values given for the DC current gain on the datasheet and as a result cannot figure out how to find the resistor values I would need. I also understand the characteristic curve to some extent but don't know where it comes from. Is it something I draw using the DC current gain values? Is this why there are different values given? Is it something given by the manufacturer? Do I choose a resistor value to use to calculate collector current or do I use collector current from the datasheet to find my resistor value? It's like there are one too many unknowns for me to create a simple class A amplifier and I don't know where to actually start in order to start making the calculations. I know once I have one value I will easily be able to make more calculations but don't know what this value is or how to find it. For reference I am attempting to use a 5v supply and a 2n2222 transistor but I eventually want to be able to create this circuit with different sources and transistors. Thanks in advance and hopefully this isn't a dumb question.
Get this:

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3