# small sig.amp completed.

Page 3.
If this works properly I will post this particular design page in the electronics general category
for people to look at and suggest design corrections, for beginners to learn from.

This can be scrutinized by people more knowledgeable to give good design suggestions so everyone starting
out in this field can benefit from the learning in this thread.

VCC = 12V.
Freq. = 400 Hz.
Vin pk. = 10mV.
VinZout = 700 ohms.
Vout pk. = 4V.
Rload = 200 ohms.
-----------------------
I will approach this design by designing CC amp. for the output to drive the load.

Stage Q1:
Vin pk. = 4V.
Re1 = 200 ohms to match Rload.
C1 = 100uF

These next steps are done experimentally because of the low VCC with respect to Vin.
Now since Vin is very high with respect to the VCC I will make RB1 around 100 > RE1

RB1 = 22K
Now hooking my generator to the input of this CC. amp. and scoping the input and output,
I use my res.sub.box. to test resistors between base and VCC, until I get a proper waveform
close to 4V pk. across a 200 ohm load. (that value is RB2)

RB2 = 3.9K

Dynamic test:
A good waveform across Rload, the amp can only take around 4V. input before it distorts
so because of the low VCC with respect to the output swing needed, the output across the Rload
is a little more than 3.8V pk. close to design values, shoiuld I realy need a 4V. out, than
I would have to use different transistors (fet, power trans, ect..). But this is just a experimental
design.
To see how stable the stage was, due to base current loading on the divider, I did the 5 transistor
substitute on this stage and the Vb, VE, and waveform amplitude and structure remained close to same,
values throughout. No significant change in the output wave.

It passed the base current loading affect test, so this stage is somewhat stable in variations of IB.

Calculate Zin estimated around 1.2K - 2.4K using Beta from (20 - 100)
Now I have a output voltage across the Load as reqired.

The Zin is still too low for me to multistage CE amps into it yet.

So I will keep the scope at the output of this stage and design another CC. amp to cap. feed into it
using a RE2 value of 1K. ---------------------------

Stage Q2:
RE2 = 1K
RB3 = 33K
RB4 = 22K

Dynamic test:

Well when I hooked up the input signal I had to lower it's amplitude now the output signal has excellent
waveshape and the amplitude is right at 4V. pk. Even when I disconnect and connect the 200 ohm LOAD
there is hardly any noticeable change in both the amplitude and shape of waveform. Very good.

Zin is now around (6k - 10k) OHMS.
I think I have enough impedance to develope a 4V. pk signal across with a CE amp stage. -------------------------------
Stage Q3:

RC3 = 4.7K
VCQ3 = 6V.
ICQ3 = 1.27mA.
Av. = 20 no load
RE3 = 220
VRE3 = 279mV.
VRB5 = 0.98V.
RB5 = 8.2K
IRB5 = 119uA.
RB6 = 91K

static test:

VC = 5.7V.
VRB5 = 1V.

dynamic test:

Waveform shows good amplitude close to 4.5V. pk. input signal needed lowered, for that stage.
Now OVERALL performance to this point, scoping Vout. at the RLOAD and scoping Vin aqt stage Q3,
shows excellent waveform and amplitude rides comfortably at a peek of 4V. across the 200 ohm load.

Disconnecting the load gives No real significant change in the Vout, of amp, and Vin is at 300mV.

I changed the transistors in all 3 stages 3 times and got the ame output waveform.
Good amplifying so far.

So 3 stages Av. is around (4V / 300mV) = 13.3
Zin = (2.7K - 5.6K) Beta (20 -100) ------------------------
Stage Q4:

RC4 = 2K
VCQ4 = 6V
ICQ4 = 3mA.
Av. = 20 no load
RE4 = 100
VRE4 = 300mV.
VRB7 = 1V.
RB7 = 6.8K
IRB7 = 147uA
RB8 = 75K
static test:
VC = 7V
VRB7 = 0.97V

Dynamic test:
Good signal waveform and amplitude:
COMPLETED AND WORKS GOOD.
----------------------------------------------
OVERALL PERFORMANCE
Vin = 8mV
Vout = 4V
RLOAD = 200 ohms.
When I connect or disconnect the LOAD there is NO apparent change in Vout amplitude nor waveform shape.
Av. = 500

When I got to the last stage and connected it to the rest of the amp, I got a lot of rf noise on the scope
I put in a 0.1 uf cap from the base of Q3 to ground to filter the noise and got a nice strong waveform,
however I was getting 4 Volt. out with 20mV. input, so I placed the AC bypass across emitter res.RE3 to bring
a nice 4V. output with a 8mV. input. So I was able to recover the gain needed to put 4.Vout @ 10.mVin.

No feedback capacitors were used, therefore a very slight phase shift from input to output.

Changed every transistor with different ones, couple times and get the same output waveform.

I designed it with my generator set at 400Hz.
So this amp is more like a narrow bandpass amplifier it only amplifies at peek voltage from 400 to around 900 Hz.
It amplifies about a 500 HZ bandwith very narrow.

This is a experiment in trying to design using a logical approach of attempting to amplify a small signal
from a impedance mismatch between input and output from a high impedance 700 ohms to a low 200 ohms output.

With a Vcc that is just large enough to handle the signal voltage of 4V. peek.

Also using NPN 2n3904 transistors throughout the design.

Vout at the RLOAD is 4V. pk. Author
hobbyist
Views
1,069
Last update