I have been given the task of designing a one-transistor amplifier to satisfy the following specification:

Voltage gain: at least 20 dB at mid-band into 600 ohms
Frequency response:
Lower cut-off frequency: 40 Hz
Upper cut-off frequency: 1 MHz
Input impedance: 600 ohms ± 10%
Output impedance: 600 ohms ± 10%
DC supply voltage: 12 V
DC supply current: not more than 15 mA
Output power: not less than 2.0 mW into 600 ohms at mid-band.
The circuit should be based on a 2N2222A bipolar transistor

The circuit is to be fed from a source that has an output impedance of 600 ohms
load of 600 ohms

I have decided on the following setup

I've done a design, but I feel several things are wrong because my resistances are tiny.

First I assumed a quiescent input current ~ 8mA = IC (are these reasonable and how to improve on them??)
quiescent collector Voltage ~ 7V = VC

VCC - DC supply voltage
VE - quiescent emitter Voltage
VBE - voltage of base relative to emitter ~ 0.7V
Rc - collector resistor
Re - Emitter resistor
RL = load resistance = 600 ohms
RS = Source resistance = 600 ohms

VC = VCC - IC*Rc
giving me RC = 625 ohms - so i would use RC = 680 ohms
VC = 6.6v

then I considered the gain, decided on a gain of 10. Including the RL in the gain calculation:

voltage gain = -(Rc//RL)/(Re)
therefore
Re = (Rc//RL)/10 = ((680^-1)+(600^-1))^-1 /10 = 31.9 ohms (this value seems very small to me, so i can tell i've done something wrong). following the calculation I would use a 12 ohm resistor because 10 is the minimum gain, therefore:

voltage gain = -(Rc//RL)/(Re) = -(((680^-1)+(600^-1))^-1 /12= 26.6 gain

next I try and find values for Rb1 and Rb2.

VE = IE*Re= 0.008*12 = 0.1 V

Biasing the base: VE + VBE = 0.1+0.7 = 0.8 V

using the general rule Rb2 ~ 10*Re = 120 ohms

VCC*Rb2/(Rb1+Rb2) = 0.8

Rb1 = (Rb2*(VCC-0.8))/0.8 = (120*(12-0.8))/0.8 = 1680 ohms, use 1.5kohm resistor

I can't help feeling I'm making silly mistakes... how do I check the amplifier meets the specification? I have spent so much time trying to understand this, I really need your help, Thank you...

Stonecrow

 

note that the values on the diagram are not the values I plan to use, and I only plan to use a capacitor on the input

 

What is the maximum peak-to-peak signal swing you are looking to be able to provide at the output? or put another way, What is the maximum peak-to-peak input signal you expect to occur at the input to your CE amplifier?

 

What is the maximum peak-to-peak signal swing you are looking to be able to provide at the output? or put another way, What is the maximum peak-to-peak input signal you expect to occur at the input to your CE amplifier?

A maximum output peak-to-peak signal swing ~8V, so that would means it needs to go above and below it's quiescent value by 4V. Can someone please explain how this changes things?
Thanks once again

 

Knowledge of the output signal swing is used to make sure that the quiescent dc collector voltage is chosen so as to optimize this setpoint to minimize any distortion that would take place if the signal swings too positive or too negative. An 8 volt peak to peak signal swing at the collector should be reasonably easy to accommodate with the 12 volt dc power source you have been given.


hgmjr

 

Ok that makes things clearer. If I have a peak-to-peak output swing of 8V, the output voltage must be able to go above and below the quiescent voltage by 4V. In order to leave a reasonable voltage across the emitter resistor I would choose the quiescent output voltage to be about 4.5V. So:

VCC = VCC – 4.5 = 12 - 4.5 = 7.5V

VC = VCC - IC*Rc

IC*Rc = 4.5V

To find Rc, I need to choose a value for the collector current (IC), and I am not really sure how. I think I could if I had the output characteristics for the 2N2222A in the form below.

However I can’t find the output characteristics for the 2N2222A!!! Does anyone know where I can find the information, or have it already?? In short I need help deciding on IC..

Cheers

 

However I can’t find the output characteristics for the 2N2222A!!! Does anyone know where I can find the information, or have it already?? In short I need help deciding on IC..

Following an extensive search of the datasheet sources with which I am familiar, I came up empty handed.

Unless someone else can locate a source of a plot of Ic versus Vce for the 2N2222A, you will just have to trudge on without that design aide.

From the calculations that you have already provided, you appear to have a fairly good grip on the general concept.

hgmjr

 

Usually, the quiescent current is chosen so that the output current doesn't affect the q-point that much. As far as I know, many choose the Ic at about 10 times the Io. Larger value is better, but the power draw is bigger. This is why for substantial current output usually an emitter follower is used.

 

Thanks to everyone for the help, I'll post what I come up with from your comments. This place rocks!

 

If there is a specification you need, contact several manufacturers of that part. Then post the one that actually sends you the data so we know which one to buy.

I've had very good luck with National and Dallas, if they have it they'll send it.

 

Here's Fairchild's datasheet

http://w3.id.tue.nl/fileadmin/id/objects/E-Atelier/doc/Datasheets/Transistoren/PN2222A.pdf

Here's 10 more manufacturer's datasheets:

http://www.datasheetarchive.com/search.php?q=2n2222A&sType=part&ExactDS=Starts

Have fun.

 

Hi,

Just to chime in - use 2N2222's in the TO-18 package. They always outperform the 2N2222A in TO-92.