opamp design

Thread Starter

prescott2006

Joined Nov 8, 2008
72
what is the gain of this circuit?does the current sink provide 200uA in this case?how to modify to obtain other desire current source?
 

Ron H

Joined Apr 14, 2005
7,063
what is the gain of this circuit?does the current sink provide 200uA in this case?how to modify to obtain other desire current source?
Change R4 to 3k to get Ic(Q8)≈200μA. Ic(Q8)≈Vbe(Q9)/R4. Vbe(Q4)≈0.6V.
I measure the gain as 101*Vout/Vx=101*Vout/Vx=101*981mV/8.93mV=11095.
All these measurements come from simulating the LTspice I posted, but with R4=3k.
What circuit did you simulate?
 

Ron H

Joined Apr 14, 2005
7,063
It's rude to post a circuit someone gave you, to another forum without mentioning the source.:mad:

EDIT: Upon further reflection, perhaps you didn't understand the etiquette of this situation. Now you do.
 
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Thread Starter

prescott2006

Joined Nov 8, 2008
72
finally i enter the lab to verify the circuit, but no luck though. cannot get any result.i have include several suggestion and comment made by my lecturer. see the attachment. so i would like to ask you whether those suggestions make sense or not?
most weird thing is when i feed some signal let say 1 Vp at the +ve input,but when i check the +ve input using oscilloscope only mV range i get. not sure what is going on.
second thing is the lab assistant say there should be 0 V at the Vo if 0V is connect in common mode to input. so i connect the +ve and -ve input and ground it, but the output transistor burn.
 

Attachments

Ron H

Joined Apr 14, 2005
7,063
finally i enter the lab to verify the circuit, but no luck though. cannot get any result.i have include several suggestion and comment made by my lecturer. see the attachment. so i would like to ask you whether those suggestions make sense or not?
most weird thing is when i feed some signal let say 1 Vp at the +ve input,but when i check the +ve input using oscilloscope only mV range i get. not sure what is going on.
second thing is the lab assistant say there should be 0 V at the Vo if 0V is connect in common mode to input. so i connect the +ve and -ve input and ground it, but the output transistor burn.
Are you aware that the pinout of the two transistors are different? If you have the pins connected incorrectly, the transistors may burn up. See attachment.

Due to the high gain and small but finite input offset voltage, you cannot make meaningful measurements (including DC offset voltage) on an op amp when it is running open loop. I have showed you how to measure open loop gain in a closed loop mode. To measure offset voltage, refer to the article I linked to previously.
You do not need emitter resistors on the input stage. They will lower the open loop gain, while not improving anything else.
You can replace R1 with a current sink, raising the simulated open loop gain to about 80000. It will also improve the output range on the negative side for heavy loads. I didn't add it originally because you didn't need that much gain. See attachment.
 

Attachments

Thread Starter

prescott2006

Joined Nov 8, 2008
72
oh,the pin out is different for each of them. i didn't aware about that.i use same pin out as 2n2907.no wonder it will burn up.they didn't provide data sheet so only can make guessing.one more question,how do you actually knowing what value should be substitute for each of them?got any formula or based on experience or based on simulation?
 

Ron H

Joined Apr 14, 2005
7,063
oh,the pin out is different for each of them. i didn't aware about that.i use same pin out as 2n2907.no wonder it will burn up.they didn't provide data sheet so only can make guessing.
I didn't know the pinout. I looked up the datasheet.:rolleyes:
one more question,how do you actually knowing what value should be substitute for each of them?got any formula or based on experience or based on simulation?
I don't know what this means. What do you mean by "them"? Do you mean resistors?
 

Thread Starter

prescott2006

Joined Nov 8, 2008
72
what is the purpose of 100nF capacitor at vcc and vee?can you send me the spice model of pn2907 to me?i didn't have it in my ltspice library.
 
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Ron H

Joined Apr 14, 2005
7,063
what is the purpose of 100nF capacitor at vcc and vee?can you send me the spice model of pn2907 to me?i didn't have it in my ltspice library.
The power supply bypass capacitors do nothing in a simulation, because they are in parallel with batteries, which have zero impedance. In the real world (e.g., a breadboard, or a PC board), the power supply wiring and PC board traces have series resistance and inductance. These can cause undesirable effects in electronic circuits (not just op amps). Power supply rails should always be bypassed on the circuit board. In circuits with multiple ICs or discrete circuits, you will probably need multiple bypass capacitors for optimum performance. Common practice is to decouple each IC. In your circuit, there are no ICs, but the op amp still needs to be decoupled.

I think the library has 2N2907. You can use it, or if you want to add the PN2907 model to the standard.bjt file, here it is:

Rich (BB code):
.model PN2907 PNP(Is=650.6E-18 Xti=3 Eg=1.11 Vaf=115.7 Bf=231.7 Ne=1.829 Ise=54.81f Ikf=1.079 Xtb=1.5 Br=3.563 Nc=2 Isc=0 Ikr=0 Rc=.715 Cjc=14.76p Mjc=.5383 Vjc=.75 Fc=.5 Cje=19.82p Mje=.3357 Vje=.75 Tr=111.3n Tf=603.7p Itf=.65 Vtf=5 Xtf=1.7 Rb=10 mfg=Fairchild)
 
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