I'm a third year student of BSECE course here in the Philippines, and one of our project is to construct an amplifier with transistor (must be NPN transistor), and the voltage gain that we must get is 40 with Vcc as 12V.

I used BC337-25 transistor then I get it's data from a datasheet. According to it, the beta ranges between 150-400. In other words, I don't know exactly what the beta is, in order to use the formulas given to us.
The only solution that I did was to costruct a circuit (it's a fixed bias amplifier considering no load), I used 12Mohms for the Rb and 1kohm for Rc. Then Use the formulas to find the beta first. After constructing the circuit, I used function generator together with an oscilloscope to check the V input and V output signal produced to see whether it really amplifies or not, and if it amplifies, to get the voltage gain of the circuit I've made. The Gain is only 7.

I substitute all the data to these formulas

Av=Rc/re

re=26mV/Ie

Vcc-IbRb-Vbe=0

Ie=(β + 1)Ib

where

Vcc=12v

Rb=12Mohms

Rc=1kohms

Av=7

Solving it gives beta as approximately equal to 205. (Not that upon performing the experiment, I only used a bread board so that it will be easier for me to construct another circuit).

Retaining Rb and using Av=40 and beta which is 205, and using the formulas mentioned, I computed Rc to be 5.7kohms. The actual gain I got using function generator and oscilloscope is 40-42.

But the project must be placed in PCB! So I design a circuit in PCB, and solder all the components in it. After that, I test it again, and to my surprise, the voltage gain increased to 62! I compute the new beta and it became 330. So again I compute th Rc retaining all other components. I got 3.5kohms. So I replaced the Rb in the PCB. Desolder it, then solder the new Rb. To my big surprise when our professor checked the gain, it's only 30-33.

I really do not know what's happening. I just conclude to my self that soldering transistor to PCB could affect it's beta, and if that's true, how can I know the new beta of the transistor in the PCB after soldering? Even your not soldering the tips of the component, it's still connected to a single circuit and it could still affect the transistor (since the circuit that I have made is very little about 1.5cmx2.5cm size of the PCB).

Please help me... We are currently making a new project, a voltage divider bias amplifier. Thank you.

 

.....
I used BC337-25 transistor then I get it's data from a datasheet. According to it, the beta ranges between 150-400. In other words, I don't know exactly what the beta is, in order to use the formulas given to us.
The only solution that I did was to costruct a circuit (it's a fixed bias amplifier considering no load), I used 12Mohms for the Rb and 1kohm for Rc. Then Use the formulas to find the beta first. After constructing the circuit, I used function generator together with an oscilloscope to check the V input and V output signal produced to see whether it really amplifies or not, and if it amplifies, to get the voltage gain of the circuit I've made. The Gain is only 7.

....

Perhaps, you are not doing anything wrong. Have you considered that? Maybe those are the results you are supposed to get. Soon you will be using the voltage divider method of biasing. mmmm..... why is another method needed?

I'd like to point out a few facts that you should keep in mind as you move forward in this learning process.

1. The formula for gain has re=26mV/Ie, however, the 26 mV is really the thermal voltage (sometimes called Vt=kT/q) where k is Boltzmann constant and T is temp in K and q is electron charge. So the gain is going to be very temperature sensitive.

2. The gain is also dependent on the dc bias current Ie which is highly dependent on beta with your circuit design. Transistors have wide range for beta, so if you try to use this design in production, every circuit you make will be vastly different in performance.

3. Your Rb is 12 M which is pretty high. When soldering, make sure you clean any flux and debris off the board. It doesn't take much to change the effective resistance of a 12M resistor. Just touching it will change the results. (not that that's is your problem, but just a good thing to be aware of)

4. As far as the idea that soldering changes beta, probably not; at least, not permanently. Beta is temperature sensitive, so if you test right after you solder, the results will be different.

The bottom line here is that you have constructed a circuit that is sensitive to temperature and beta changes. Clearly, your prof. is trying to make you learn with hands-on struggling, which is very effective!

 

Rosin flux will "bite" you if you do not thoroughly clean it off the board using isopropyl alcohol or the like, particularly when you are dealing with very high resistances such as your 12M resistor. The flux has a fairly high resistance, but when put in parallel with a very high resistance will change the electrical characteristics of the circuit.

 

Perhaps, you are not doing anything wrong. Have you considered that? Maybe those are the results you are supposed to get. Soon you will be using the voltage divider method of biasing. mmmm..... why is another method needed?

I'd like to point out a few facts that you should keep in mind as you move forward in this learning process.

1. The formula for gain has re=26mV/Ie, however, the 26 mV is really the thermal voltage (sometimes called Vt=kT/q) where k is Boltzmann constant and T is temp in K and q is electron charge. So the gain is going to be very temperature sensitive.

2. The gain is also dependent on the dc bias current Ie which is highly dependent on beta with your circuit design. Transistors have wide range for beta, so if you try to use this design in production, every circuit you make will be vastly different in performance.

3. Your Rb is 12 M which is pretty high. When soldering, make sure you clean any flux and debris off the board. It doesn't take much to change the effective resistance of a 12M resistor. Just touching it will change the results. (not that that's is your problem, but just a good thing to be aware of)

4. As far as the idea that soldering changes beta, probably not; at least, not permanently. Beta is temperature sensitive, so if you test right after you solder, the results will be different.

The bottom line here is that you have constructed a circuit that is sensitive to temperature and beta changes. Clearly, your prof. is trying to make you learn with hands-on struggling, which is very effective!

Thank you very much for your help sir!

Well, after doing the next project, which is the voltage divider bias, we will going to compare it to the previous one. To know which is more stable, advantages and disadvantages etc...

I forgot to tell sir that one of the categories in the past project is that the value of Rc must be 1kohms-6kohms only. If i'm going to decrease Rb to around 500kohms, it will be impossible for me to have Rc between 1kohm-6kohms because the resulting gain will be too low, and it doesn't fit with the gain that we must have.

Another thing is that, I'm finding values of Rb in megaohms (between 5Mohms-10Mohms) because that's the one I need to make Rc be in 1kohm-6kohms range, but unfortunately, Megaohm resistors are difficult to find here in our place. Available Megaohm resistor is only 2megaohms. I find 12Mohms in one electronic shop and that's my only choice so i used it. I didn't know the fact that very high resistance will be sensitive when soldering it to PCB.

Thank you.

 

Rosin flux will "bite" you if you do not thoroughly clean it off the board using isopropyl alcohol or the like, particularly when you are dealing with very high resistances such as your 12M resistor. The flux has a fairly high resistance, but when put in parallel with a very high resistance will change the electrical characteristics of the circuit.

Sir what do you by "Rosin flux will "bite" you"? Yup I clean it with alcohol after soldering.

Thank you sir for your reply.

 

Can you post your schematic? Transistor circuits should be designed to be largely immune to beta variations and it sounds like yours isn't. I think the implementation may be incorrect.

 

Rosin flux is an acid dissolved in grease. That's why it stings if you touch it.
It's purpose is to remove the oxide layer that is present on the surface of all metals.
This exposes a fresh metal surface for the solder to bond to.
The solder used for electrical purposes normally includes this flux.

As regards to your circuit and course, Steve has hit the target smack on, your teacher is trying to make you learn by asking questions - and doing vey well.

Questions to ask yourself

What would happen if you increased the supply voltage?
What would happen if you increased the collector resistor?
12 meg resistors are not common as you have found out so can you think of any better circuit arrangement to supply the base. You have had the hint with the words voltage divider.

Good luck with your studies.