Hi,
A simple bjt won't work satisfactory.
There are Opamps, they can do it almost perfectly.
I'm not sure about your requirement.
* You say input range is <1 - 2V>. Does it mean that the lowest expectable input voltage is +1V and the highest expectable input voltage is +2V?
* ...similar with output voltage.
* You also say: becouse I need also inverse this signal.
Does this mean you need a second output signal?
An does it mean the output voltage should go from -1V to -7V?
A picture, a diagram, a formula tells more than a lot of words.
Klaus
Hi,
it looks like you are interested in amplifying an rectified AC signal. So as mentioned, you are interested in DC signals only.
Where is the output of your circuit? I assume it is it the BJT collector, as you mentioned the output signal is inverted.
- You could apply your rectification circuit at the output of your common emitter amplifier.
- AC couple your input by means of a capacitor, if your signal is really an AC signal.
- Bias your transistor by the resistive voltage divider as shown in the left circuit in your attached image.
Doing so, you have not to worry if your input signal is large enough to turn your BJT on.
greets
Hi,
Please understand that we only know what you show us. We don't have the background knowledge that you have.
So please explain:
* you say your input voltage range is +1V ... +2V, but the given diagran shows -0.7V .... +1.8V
* still unclear what "i need only the inverse signal" means.
Klaus
Hi,
O.K. now it's much clearer.
I don't know what accuracy and precision you want..
Fir my tastet the value of C2 is much too high. It needs a lot of current to get charged. Then with R3 you get a too high time constant.
Thus you can't catch the peak voltage of R1.
With a smaller value of C2 you'd get a higher voltage at C2, since it's much closer to the peak voltage of R.
If you use a small schottky diode at D1
But then you also need a higher value of R4 .... not to discharge C2 too fast.
Currently you have a time constant of 64ms.
The higher the better.
Now to the end amplifier:
If you want an input range of 1V (+1V ... +2V) at C2 to become 6V (+7V ...+1V) at the output you need a gain of -6.
Usually you may estimate small signal voltage gain to be -R7/R5.
The base current will reduce this gain. For a lower base current you need a smaller collector (emitter) current.
For an almost ideal (one bjt) voltage amplifier the voltage at R5 is V_C2 - 0.55V --> 0.45V ... 1.45V
The voltage drop (at gain of 6) at R7 then would be 6 x V_R5 --> 2.7V ... 8.7V
With your R12 this is impossible.
Even if you omit R12 the output voltage then should be 8V - V_R7 --> impossible. (8V - 8.7V = negative)
Let's try to calculate backwards:
Output voltage 7V down to 1V needs a R7 voltage 8V - V_out of 1V up to 7V at R7.
This needs a R5 voltage of 0.67V .... 1.67V.
And here we see the biggest problem. You can't get 1V output voltage with V_R5 to be 1.67V.
The output voltage (at emitter) never can be lower than the R5 voltage (at collector)
And this is why I underlined the words "small signal" before. You are far away from small signal operation.
Your output signal is large ... indeed it is clipped.
--> I see no good solution to get your desired output signal with the current circuit.
You need some major modifications in your circuit. Less output voltage range, higher supply voltage...
Klaus
Hi,
my solution would be a tiny microcontroller with ADC.
I will
* generate the excitation signal (your astable multivibrator) with software adjustable frequency and timing, and extreme XTAL precision
* catch the signal with most precision (no rectifier needed, no peak detector needed), no delay, perfectly tracking the peak, easily to synchronize
* and it will perform additional functions (like your comparator) with perfect thresholds, adjustable, and adjustable hysteresis.
It will replace 17 parts of your schematic while increasing precision, speed, decreasing cost, no internal drifts, ...and providing most flexible adjustment per software.
But you will have your reason for your circuit.
*******
I won´t calculte the whole circuit for you.
But you may try:
* Omit D2 and R12 (to increase output voltage range)
* C2 = 470n (to get better track of peak)
* R4 = 160k (to keep your decay time constant)
* Q2 = BC547C (to reduce BOM, and adjust on gain and current)
* R5 = 2k (to decrease base current)
* R7 = 15k (to get expected gain)
No guarantee. Your simulation will show the results.
You are still in large signal area, htus expect unliearity.
And you will have thermal drifts, and drift with time, and there willl be bad PSRR...and there will be a lot of ripple (from one peak to next peak)
All this will decrease precision.
--> OPAMPS are much better. Microcontroller even better.
********
Precision = repeatability. It can´t be calibrated.
You may calibrate for accuracy.
Klaus
output from 2.5V to 7.5V. I need to make down limit lower, 2.5V to lets say 1.5V.
Your output stage appears to be a class A amplifier. At no time does it provide low resistance to either supply rail, therefore you have reduced range of output voltage.
If you want larger output swings, then you want an amplifier that provides low resistance to one supply rail, while causing high resistance to the other supply rail. (And vice-versa.)
Possibly a half-bridge. This is contained in an op amp, by the way. The type with rail-to-rail output.
no way.Lets say the allowed precision is 100mV in 2 to 7V range.
Your schemtic of post#6 showed a capacitor of 4uF and a discharge resistor of 16k this gives a tau of 64ms.-regarding to R4 = 160k, i need a time response to 100ms (i didnt mention), R4 must be lower
I recognize that I don´t know enough about your requirements to answer this.is it good aproach?
Hi,
No guarantee. Your simulation will show the results.
You are still in large signal area, htus expect unliearity.
And you will have thermal drifts, and drift with time, and there willl be bad PSRR...and there will be a lot of ripple (from one peak to next peak)
All this will decrease precision.
--> OPAMPS are much better. Microcontroller even better.
********
Precision = repeatability. It can´t be calibrated.
You may calibrate for accuracy.
Klaus
I think class A is good for this purpose.
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