# Please help transistor amplifier

by michael1978
Tags: amplifier, transistor
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 P: 410 We need Re2 resistor simply because it is almost impassible to build amplifier with desired low voltage gain and Ve > Vbe. Which is needed for good bias point stability. But if you determined to use only Re1 resistor. We need to change DC bias method. So we are not going to use voltage divider, instead of voltage divider we will use a collector-feedback bias circuit. So for our example for Vcc = 10V and Rc = 1K;RL = 10K; Ic = 5mA and Av = 50. re = 26mV/Ic = 5.2Ω Re1 = (Rc||RL)/Av - re = 909Ω/50 - 5.2 = 12Ω Vc = Vcc - Ic*Rc = 5V Vb = Ic*Re1 + Vbe = 0.71V And Ib = Ic/hfe = 5mA/150 = 33.4μA Rb1+Rb2 = (Vc - Vb)/Ib = 4.29V/33.4μA = 128KΩ Rb1 = Rb2 = 128K/2 = 68KΩ And simulation show that voltage gain is equal to AV = 51.8092[V/V] Attached Thumbnails
P: 133
 Quote by Jony130 We need Re2 resistor simply because it is almost impassible to build amplifier with desired low voltage gain and Ve > Vbe. Which is needed for good bias point stability. But if you determined to use only Re1 resistor. We need to change DC bias method. So we are not going to use voltage divider, instead of voltage divider we will use a collector-feedback bias circuit. So for our example for Vcc = 10V and Rc = 1K;RL = 10K; Ic = 5mA and Av = 50. re = 26mV/Ic = 5.2Ω Re1 = (Rc||RL)/Av - re = 909Ω/50 - 5.2 = 12Ω Vc = Vcc - Ic*Rc = 5V Vb = Ic*Re1 + Vbe = 0.71V And Ib = Ic/hfe = 5mA/150 = 33.4μA Rb1+Rb2 = (Vc - Vb)/Ib = 4.29V/33.4μA = 128KΩ Rb1 = Rb2 = 128K/2 = 68KΩ And simulation show that voltage gain is equal to AV = 51.8092[V/V]
JONEY thank you for reply i am learning a lot from you, you are the beste;-) COMPLIMENT
P: 133
 Quote by yungman So you know it is 1.7V at the base and 1V at the emitter. YES, you set Ie with the emitter resistor ( Re1+Re2). This is the KEY of DC biasing. You first set up the operating current Ie of the transistor, this is the first and foremost thing. Collector don't set the current of the BJT, collector current is only follow the emitter current. AGAIN, this is in Malvino. 1) In the diagram, Re1+Re2 is about 200 ohm. You put 1V at emitter, you put 1V across the Re1 and Re2. SO you force 5mA through the resistors......Which, means you set the Ie to 5mA. This is how you set up the DC bias current. 2) After setting the Ie, then you start looking at the collector. You know β is high, so you know the Ic≈5mA. If you have Vcc=10V, and you have 1K resistor at the collector, then you know 5mA through 1K is 5V. So the collector is at +5V. 3) AGAIN, gain is determined by the impedance at the collector divided by the impedance at the emitter. We gone over this over and over and over already. Read the old posts AGAIN. These are all in Malvino. Please read that before you ask any more question. If you don't get it, read it, write out the numbers, work through the problems before you post.

did you see the post 146 of Joney, maybe i am mistake but malvino he dont show you how to get desired voltage, i wil like to ask you why Jony, he put so big resistance of voltage divider i say for r1 22K and for R2 4.7k how he calculate,? me i put it R2=170, and R1=830, look how big difference, he show one example but me i put in other way,
P: 3,898
 Quote by michael1978 did you see the post 146 of Joney, maybe i am mistake but he dont show you how to get desired voltage, i wil like to ask you why Jony, he put so big resistance of voltage divider i say for r1 22K and for R2 4.7k how he calculate,? me i put it R2=170, and R1=830, look how big difference, he show one example but me i put in other way,
We were talking about the diagram of post 114 all along until now.

The newer diagram is not as desirable. I believe it called self bias or something. It depend a lot on the beta of the transistor. Voltage divider bias is a better way to go.
P: 133
 Quote by yungman We were talking about the diagram of post 114 all along until now.
yes but i say to joney i gonna make one amplifier, and he help answer me thanks from him, ok why Joney select so big resistance of R1 22k and R2 4.7k
and he get the sam base almost 1.7V, how i have to do it big resistance like joney
P: 3,898
 Quote by michael1978 yes but i say to joney i gonna make one amplifier, and he help answer me thanks from him, ok why Joney select so big resistance of R1 22k and R2 4.7k and he get the sam base almost 1.7V, how i have to do it big resistance like joney
You talking about post 114? The resistance is 22K//4.7K ( if I ignore the input resistance of the transistor). In fact the input resistance is quite low to me.

Usually you want the input resistance to be a little on the high side so the stage driving this input don't have to drive a low impedance. Think if you have a stage like in post 114, if the collector is driving the following stage with low impedance like 1K. The collector resistance is 1K//1K=500 ohm. You lower the gain of the driving stage.

In both diagrams, the input resistance of the transistor is β(r'e + Re1).=β(20Ω)≈2K assuming β=100. This is quite low. Another way to look at it is the transistor in both case require 5mA/β= 50uA. The impedance of the input biasing network has to be low enough so it is not loaded down by the input of the transistor. Jony got into this a little in post 79 about the base current.
P: 133
 Quote by yungman You talking about post 114? The resistance is 22K//4.7K ( if I ignore the input resistance of the transistor). In fact the input resistance is quite low to me. Usually you want the input resistance to be a little on the high side so the stage driving this input don't have to drive a low impedance. Think if you have a stage like in post 114, if the collector is driving the following stage with low impedance like 1K. The collector resistance is 1K//1K=500 ohm. You lower the gain of the driving stage. In both diagrams, the input resistance of the transistor is β(r'e + Re1).=β(20Ω)≈2K assuming β=100. This is quite low. Another way to look at it is the transistor in both case require 5mA/β= 50uA. The impedance of the input biasing network has to be low enough so it is not loaded down by the input of the transistor. Jony got into this a little in post 79 about the base current.
CAN I ASK YOU SOMETHING about impedance, my english is not so good, how you select input impedance, not to say in the end the input impedance was 50k of 10k etc, you select in the begin of how
 P: 3,898 For normal frequency ( not RF), you usually want it to be as high as practical. It is limited by the input requirement of the amplifier.........Like in your case, the transistor Ib of 50uA and transistor input impedance. Jony in post 79 explained this and I explained about the input impedance in the post 150. In your case, with drawing in both post 114 and 146, the emitter resistor Re1 is eithe 14 or 12Ω, that is very low. That is really the gating factor of the circuit. Also you are trying to get gain of 50 out of one transistor, this force you to have such a low Re1 in order to be able to use a collector resistor of 1K. In real life design, if I want to have a gain of 50, I would do either one below: 1) Using Re1=150, Rc=10K, Ic=0.5mA, then use an emitter follower transistor to buffer the output. 2) Use two stage of this and divide the gain between the two stages. This is the prefer way. 3) Use JFET instead of BJT.
P: 133
 Quote by yungman For normal frequency ( not RF), you usually want it to be as high as practical. It is limited by the input requirement of the amplifier.........Like in your case, the transistor Ib of 50uA and transistor input impedance. Jony in post 79 explained this and I explained about the input impedance in the post 150. In your case, with drawing in both post 114 and 146, the emitter resistor Re1 is eithe 14 or 12Ω, that is very low. That is really the gating factor of the circuit. Also you are trying to get gain of 50 out of one transistor, this force you to have such a low Re1 in order to be able to use a collector resistor of 1K. In real life design, if I want to have a gain of 50, I would do either one below: 1) Using Re1=150, Rc=10K, Ic=0.5mA, then use an emitter follower transistor to buffer the output. 2) Use two stage of this and divide the gain between the two stages. This is the prefer way. 3) Use JFET instead of BJT.

thank you for answer, and R1, R2? but first i am bussy with transistor, is difficult jfet to learn, and for the book, you can buy it chip used boooks in amazone, because i want to open one visa of mastercard
P: 3,898
 Quote by michael1978 thank you for answer, and R1, R2? but first i am bussy with transistor, is difficult jfet to learn, and for the book, you can buy it chip used boooks in amazone, because i want to open one visa of mastercard
When you use a credit card to open an account with Amazon, make sure don't let your computer remember your password, and Amazon ask you whether you want to keep logging in even you close the page......DON'T. Always logout when you close the page and don't let the computer remember your pass word.
P: 133
 Quote by yungman When you use a credit card to open an account with Amazon, make sure don't let your computer remember your password, and Amazon ask you whether you want to keep logging in even you close the page......DON'T. Always logout when you close the page and don't let the computer remember your pass word.
yeh i know but there are a lot of hackers today, i have to open only with 200E if it is possible, because i think they will not open with low credit
P: 133
 Quote by michael1978 JONEY thank you for reply i am learning a lot from you, you are the beste;-) COMPLIMENT
HI Jony how are you, may i ask you something, the schematic of designin of amplifier , you make it with light spice of some other software , because you told me i use light spice,
greetings
P: 3,898
 Quote by michael1978 yeh i know but there are a lot of hackers today, i have to open only with 200E if it is possible, because i think they will not open with low credit
Well, there is a balance of being safe and missing out. Write to Amazon and ask what is the best way. It is their utmost interest to keep it safe as their business depends on this too.
P: 133
 Quote by yungman Well, there is a balance of being safe and missing out. Write to Amazon and ask what is the best way. It is their utmost interest to keep it safe as their business depends on this too.
thnx for reply, yes i know, but i will go first to my bank to ask
 P: 3,898 The bank cannot help much, call the credit card company like VISA also. Ask Amazon, they should be able to help you. I have very good experience with Amazon, I have been ordering from them for years, never have a single problem. Their service is second to none......absolutely none. One time I order a tiller.....over 100lbs gas tiller for turning the ground, There was a small crack on some non critical area, I asked for exchange, they shipped me a second one immediately and arranged to pickup the first one. The kicker was I decided to keep the original, they said "no problem", they just arranged to pickup the second one!!!! I buy computers, guitar pickups, protein powders, gifts.....I just paid $1600 to buy a new Nikon camera and Tamron lens from them just a month ago. I even paid a little more just to buy on Amazon because of their service. Oh Yeh, thousands of dollars of text books used from Amazon also. When they say is good condition, they ARE in good condition. Half the time, they look new. One thing about studying, you need books, lots of books. I buy 7 to 8 text books on each subject to compare as no book is good in all topics, each has their strong and weak points. You need a few to get the whole picture. Malvino is one exceptional one. Can you imagine if I buy my two tall book shelves of new books?!!! That would have been very expensive. I just bought the Malvino 6th edition for$8 shipped!!!!
P: 133
 Quote by yungman The bank cannot help much, call the credit card company like VISA also. Ask Amazon, they should be able to help you. I have very good experience with Amazon, I have been ordering from them for years, never have a single problem. Their service is second to none......absolutely none. One time I order a tiller.....over 100lbs gas tiller for turning the ground, There was a small crack on some non critical area, I asked for exchange, they shipped me a second one immediately and arranged to pickup the first one. The kicker was I decided to keep the original, they said "no problem", they just arranged to pickup the second one!!!! I buy computers, guitar pickups, protein powders, gifts.....I just paid $1600 to buy a new Nikon camera and Tamron lens from them just a month ago. I even paid a little more just to buy on Amazon because of their service. Oh Yeh, thousands of dollars of text books used from Amazon also. When they say is good condition, they ARE in good condition. Half the time, they look new. One thing about studying, you need books, lots of books. I buy 7 to 8 text books on each subject to compare as no book is good in all topics, each has their strong and weak points. You need a few to get the whole picture. Malvino is one exceptional one. Can you imagine if I buy my two tall book shelves of new books?!!! That would have been very expensive. I just bought the Malvino 6th edition for$8 shipped!!!!
i will try, so chip you buy malvino, and do you have 7 edition, if you have 7 edition, wich is better, 6 of 7 edition,
 P: 3,898 I have not received it yet. I must had the first edition in the late 70s!!! I long lost the book. The book is in my head and been using what I learned to do a lot of designs all these years. I just want to keep a copy for my collection, not that I need it. I since learn a whole lot more about transistors and op-amps. It is a very good introduction book for electronics.
P: 133
 Quote by yungman I have not received it yet. I must had the first edition in the late 70s!!! I long lost the book. The book is in my head and been using what I learned to do a lot of designs all these years. I just want to keep a copy for my collection, not that I need it. I since learn a whole lot more about transistors and op-amps. It is a very good introduction book for electronics.
you can find it in internet

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