# Transistor Circuit Design - confusing with the datasheet.

#### mityeltu

Joined Dec 18, 2015
3
Ok, I'm not new to electronics, but I have always had alot of trouble actaully designing stuff that works. This is, I believe, due to a fundamental misunderstanding of how to interpret data found on the datasheets of the components I use. So, I did some digging and tried to give myself a bit of a tutorial. I failed.

I found the following design "guide" if you want to call it that (attached, hopefully). I can work through the math easily and understand the design, but what I got lost in was the interpretation of the datasheet. On page 2, in thr paragraph right above the bold "Ic = 2mA" statement (middle of the page) he states:

"Practically speaking, at 10 mA the electrical characteristics for the BC547Β transistor are not so great (there are also some noise issues with biasing at 10 mA). For example VCE(SAT) is 0,6 volts from 0,2 volts. From the datasheet for the BC547B we can see that for a 2 mA collector current is behaves quite nicely."

When I look at the datasheet, I don't understand how this is devined. Is there a reference/book/article that anyone knows of that can help me understand how to actually use the datasheet to make design decisions?

Please, don't misunderstand. I understand some of the stuff in the datasheet, like max current , max voltage, hfe at certain test points and the like; but if my design does not follow the specific test criteria (say, inpuit voltage of 9V as opposed to 5, or like the attached "guide" a colloctor current of 2mA as opposed to 10) I don't know what to do with what I see in the datasheet. How do I infer or interpolat/extrapolate the information I need to make my design from the parameters listed?

I hope this nis not too long or too muddy. I hope someone can help me. Thank you.

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#### MrChips

Joined Oct 2, 2009
30,433
Most transistor circuits can be designed with little reference to the data sheets at the initial design stage.

You design the circuit with the specific application in mind, such as, low/high signal, frequency, audio/RF, power, noise, linear/switching, etc.

You then need to go to the datasheets to select the appropriate current, voltage, power, gain, bandwidth, packaging, etc.

Then you can proceed to do a simulation of your circuit. Tweak the resistor and capacitor values as need.

Finally, proof of design comes with real testing. You will never know how good or bad is your design until you build the actual circuit and test it. Low frequency circuits can be tested on prototyping boards. High frequency circuits (higher than 1MHz) will benefit from proper PCB layouts even at the prototyping stage.

#### Alec_t

Joined Sep 17, 2013
14,201
Welcome to AAC!
I don't know where your pdf author got his info from, but according to this datasheet , Vce(sat) at Ic=10mA is typically 0.25V and doesn't get to 0.6V until Ic =100mA.
That aside, the amp described is designed to have the collector sitting at about half the supply voltage, so Vce(sat) is largely irrelevant. It would be relevant if instead you were using the transistor as a switch, driven fully on. Then you would want to minimise heating (Ic x Vce), so minimising Vce would make sense.

#### mityeltu

Joined Dec 18, 2015
3
I concur. I could, using an ideal model or the large/small signal equivalents design a circuit to perform some particular function, but that does not help me understand how to interpret the data on the datasheet. If I know I want an amplifier with a gain of 100, that's easy. Looking at a datasheet I can see that it has the appropriate hfe. I can then look and see if it can handle the desired Ic. I can then design the bias for the needed Ib, but again that would not help me understand if there is some odd behavior at (like the "guide" I attached) an Ic of 10mA vs the suggested 2mA. Where is this information contained? How did the authro arrive at this conclusion?

#### mityeltu

Joined Dec 18, 2015
3
@alec: That's a good point, so I should ignore that; however, the author does not appear to be attempting to minimise Vce, but rather point to a potential issue arising from noise give a bias current of 10mA. I really don't understand this. And then the reference to good beahvior at 2mA.... How is this devined from the datasheet?

#### AnalogKid

Joined Aug 1, 2013
10,943

#### Jony130

Joined Feb 17, 2009
5,484
The large the collector current is, the smaller re and R1, R2 will be. Hence the lower Zin. Which is not good in the first stage.

#### Bordodynov

Joined May 20, 2015
3,162
Sometimes the noise of the first cascade is important. There is an optimum collector current for obtaining the minimum noise factor. This current depends on the impedance of the signal source and the frequency range.

#### hobbyist

Joined Aug 10, 2008
892
Ok, I'm not new to electronics, but I have always had alot of trouble actaully designing stuff that works.
I don't know if you caught this or not, so
Just for future reference, the author used a "gm" equation for the "re" value.
"gm" is the ratio of delta input voltage to delta output collector current and is equal to {IC / 25mV}
"re" is equal to {1 / gm} which works out to {25mV / IC}

In this case of how he used the first equation {Av = RC / re} then RC would equal 8 ohms.
By the time he got to the third equation he straightened it out, to get the final correct answer.

However if you were to follow along with his three equations in a row, it could get very confusing to see how he attained the final answer.

The equation to solve for RC in this example is better to be attained as follows,
{Av = (RC / re)}
{re = (25mV / IC)} as [(25mV / 2mA)] = 12.5 ohms.
{Av = (RC / 12.5 ohms)}
{RC = (Av x 12.5 ohms)} so [ RC = (100 x 12.5 ohms)] = 1.25k ohms.