# simple question about transistor datasheet

#### baby_1

Joined Jun 3, 2011
39
hello i see the 2n3553 NPN power transistor for rf.
i see these statements

Minimum Gain: 10dB
Efficiency: 50%

what is "Gain: 10dB" meaning?
why they show us the effeciency of transistor?it doesn't related to circuit that we design? for example class A or class C?

Thanks

#### bertus

Joined Apr 5, 2008
21,681
Hello,

The gain of the transistor is depended on the frequency it is used.
In figure 8 on page 6 of the attached datasheet you will see the relationship,
You also see that the graphs go down to 50 Mhz, below this frequency the amplifier becomes instable with a great chance of oscillating.
All circuits are based on a class C amplifier.

Bertus

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#### Papabravo

Joined Feb 24, 2006
17,300
Decibels are used as a convenient means of specifying the gain of an amplifier over a wide range. The formula for "Voltage Gain" is:
Rich (BB code):
20 * log_10(Vo/Vi)
In the transistor example we can solve for the ratio of voltage out to voltage in by the following
Rich (BB code):
10 = 20 * log_10(Vo/Vi)
0.5 = log_10(Vo/Vi)
10^(0.5) = Vo/Vi
3.16 = Vo/Vi
So as an RF amplifier if the input from an antenna was 50 microvolts, the output would be 158 microvolts.

#### baby_1

Joined Jun 3, 2011
39
Thanks Papabravo
i want to ask my main question.doesn't have an effect the bias of a transistor on gian?why datasheet show us the gain?

#### bertus

Joined Apr 5, 2008
21,681
Hello,

The class C amplifier is non-linear.
You will need a certain amount of power to "activate" the amplifier.
Above this lower input level the gain will be seen.

See pages 5 and 6 of the attached PDF for more info.

Bertus

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#### Papabravo

Joined Feb 24, 2006
17,300
There are different types of bias circuits. If I am not mistaken Class C circuits are usually clamped to Vcc and only conduct for a small portion of the cycle. So why do you think the bias will have any effect?