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# Successful creation of an oscillator and class A amplifier - Schematics and explanations for rookies.

#### GreenGecko7

##### New Member
Hello !

My 1st post here was a few weeks back and I failed at creating a class A amplifier at the time. With the help of members and a lot of online browsing, I managed to edit and manufacture 2 small circuits I'm proud of and want to share here for the next beginners to be found.

1) OSCILLATOR

Why : I needed a signal source that would be reliable and would not need me to hook my precious phone to a circuit that could fry it (I fried my sound card. Twice. Never again.)

Schematic : (attached, random R and C values, mine land on 1.6 kHz. Must be adapted to the target frequency. ).

Notes : I decided to combine a simple "led blinker" circuit changing the values to aim at a 1.5kHz oscillation frequency. I then added 3 RC filtration stages in series to turn the "square" output of the oscillator into triangle and sine. Note : The RC filters should match the oscillator frequency to effectively integrate the signal.

Pros/Cons : High output impedance, but this also protects the circuit from what could backfire from the output wire. So quite useful for a test circuit indeeed.
Many waveforms in one.
Good tension swing for testing (10Vp-p on a 12V input for the square wave, 2Vp-p on the triangle, 1V p-p on the sine after 3 filters), can be lowered with a tension bridge.

Upgrades : a supply tension regulator (zener diode based would be easy to implement and suitable since power is low).

2) Class A amp

Why : To practice amplification of faint signals.

Schematic : (attached).

Notes : It is a 2-staged amplifier. First stage is a common collector with gain 10, which amplifies incoming AC tension by a factor 10 but shows poor impedance. 2nd stage is an emitter follower (common emitter) to have a more decent output impedance (measured at 18 Ohm : 6V output no load and 5.54V output with a 220 Ohm load.).
Both transistors used are BC337 NPNs.

Pros/Cons : Simple to build BUT output impedance is too high to drive 4 Ohm speakers. Also, the BC337s burn in a second when trying to output 1V peak-peak to a 4 Ohm speaker.

Upgrades : Use beefier second stage transistor, like BC120 + heatsink. Find a way to lower output impedance.

Hope you "like it" and hope it may be useful to someone someday !
I strongly recommend lectures #198, #273 and #356 of Youtuber w2aew which greatly taught and inspired me !

#### Attachments

• Class A 10 fold tension gain.jpg
1.3 MB · Views: 11
• sawtooth signal generator (ideally square).jpg
1.4 MB · Views: 10
• LTSpice drawing and actual circuit.jpg
1.4 MB · Views: 15
• Triangle waves.jpg
1.3 MB · Views: 8
7 KB · Views: 10
This is a good starter project. Just a few clarifications.

Class A is a "linear amplifier" loosely meaning it is constantly flowing current where the collector acts a high impedance current sink. i.e. It is not switched on and off, as in the case of this oscillator where each alternatively pulls low which turns off the alternate base with a negative voltage. Using 12V exceeds the safe reverse base-emitter limits Vebo = 5V max.

Also keep in mind the resistor ratios to optimize performance.

You may consider Rb/Rc = 10 typ to 30 or < hFE min

Next look at CMOS Schmitt Inverter for this "Astable Oscillator" which produces nicer square waves.

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