small signal amp

[andy257]

hi all

if someone could answer the following questions that would be great.

i am trying to make a preamp for a pcb microphone. I am doing some simulations at the mo on orcad and i am having some trouble getting what i require. I found this circuit on the web.

Now i am using a different opamp and DC voltage in the simulation because i dont have the model for the opamp it says to use in the circuit. Anyway i need the circuit to perfrom over the audio range (upto 20khz) and it must run from 3.3V or less. I imagine it will need a failry fast slew rate for audio aswel. If anyone has a good choice please let me know of the IC. the ic from the schematic says to use a TLC727.

Attached is the circuit and time analysis simulation. What i noticed is the output is fairly distorted. Is this becasue of the op-amp? i am assuming it is becasue the 741 is not that fast. Also orcad does not like open ended components therefore the output from the capacitor is tied to ground via a large resistor. Would this cause distortion? in reality i would not include that part if i was to make the circuit. its only for orcads simulation.

also although the circuit is working i was wondering why the ac signal is not fed into the + terminal of the opamp. Basically a non inverting amplifier and the ac signal fed into the +ve terminal via the two resistors in series. Ive tried that circuit and it didnt work. ive also attached my attempt at the circuit.

thanks

 

[Roff]

The apparent distortion is caused by having too few samples per cycle in your simulation. Make the minimum sample time 10usec, and then try larger values and watch the changes.
Your second circuit needs a 10uF cap in series with R7. Otherwise, the DC bias at the input (7.5V) gets amplified by 16, saturating the amp at the positive rail.
The slew rate of a 3V p-p, 20kHz sine wave is only 190mV/usec (1.9*10^5). Most op amps have no problem with this.
A 12V p-p sine wave at 20kHz will slew at about 750mV/usec.

 

[andy257]

thanks for the reply,

i am not that familiar with orcad but i cant seem to find anywhere a place to define the step time. You can specify the simulation time in seconds but not the number of steps. This is for a time domain (transient) analysis.

Ive put the cap in series now and it does look like it is working. Why do you think the circuits tend to be designed the other way? to save one cap or is it just preference. the circuit i had thought of to me seemed the most logical, unless i wanted an inverted signal. suppose it doesnt really matter for audio.

 

[audioguru]

An inverting opamp is a poor choice as a preamp for an electret microphone. The electret microphone has a fairly high impedance that is loaded down and attenuated by the low input impedance of an inverting opamp.

The input impedance of your non-inverting opamp is also low because the 10k bias rersistors could be 100k or higher.

 

[Roff]

Go **broken link removed** and do a search for "step size" (without the quotation marks).

 

[andy257]

thanks Ron i got it working. It simulates fine now.

One last question i have.

can the capacitors be any type i.e non polarised or should they be electrolitic etc.

also i was thinking of using the LM358 op amp as it is a low voltage opamp. It can operate as low as 3V which is just under my 3.3V operating limit i require. However ive seen previous post from the likes of audioguru etc saying the lm358 is crap. Is there a modern day IC with low operating voltage without the horrible crossover distortion etc.

thanks

 

[audioguru]

The MC34071 single, MC34072 dual and MC34074 quad opamps have a minimum supply voltage of 3.0V, have low distortion and a bandwidth to 100kHz. They are much better than the lousy old low power LM358 but use a supply current that is normal for most opamps.
They are not low noise so might make a noticable Hisss in a mic preamp.