Inverted output of common emitter amplifier


Joined Jan 29, 2010
Welcome to AAC.
Usually the speaker is connected via a capacitor in order to block the DC component of the signal.


Joined Oct 7, 2019
if it will cause any problems how invert the inverted output?
For audio, the signal has been inverted many times getting from microphone to recording to speaker. Most audio boxes try to invert an even number of times. (not inverted) Your eyer can not tell if audio is inverted.


Joined Oct 7, 2019
Sounds travels about 1000 feet/second.
At 1khz you will get 180 phase shift every 6 inches.
Just walking across the room you will have many phase inversions.
Your amplifier is not a problem.


Joined Aug 7, 2020
I am designing a simple common emitter amplifier as shown in the image. The output is inverted, I want to know what will happen if I directly connected a speaker to the inverted signal and if it will cause any problems how invert the inverted output?

View attachment 268369
If you are bothered, then connect the speaker reversed with the - terminal to the signal and the + terminal to ground, then the signal will be the right way round again.
(But, as everyone else says, you can't tell the difference).


Joined Oct 2, 2009
The few times that it makes a difference is when you have multiple channels or multiple loudspeakers.
If the acoustic output at the loudspeakers are not in phase you get phase cancellation.
This is very noticeable at low frequencies when two loudspeakers out-of-phase are placed right beside each other.


Joined Mar 14, 2008
What about this circuit
Similar to your circuit, it has low efficiency, high distortion, and a poor low frequency response (due to the low value output capacitor).
Use the circuit AG suggested if you want better performance.

But a proper circuit with a push-pull output, and negative feedback from the output would be much better.


Joined Oct 2, 2009
Human voice ranges from 85Hz to 255Hz
That is a misconception.
85Hz to 155Hz for a man's speaking voice and 165Hz to 255Hz for a woman's voice refer to the fundamental frequencies.
However fricatives such as s, sh, z and zh sounds can extend beyond 2000Hz. Proper reception of these sounds are critical for correct speech recognition.


Joined Oct 2, 2009
What value of cut-off frequency is required for audio?
Define "audio".

If you want to drive the neighbors crazy with a 1000W boom box in you car then go as low as you can go (1-10Hz).
Most audiophiles will be happy with 20Hz. The lowest note on my 4-string bass is 41Hz. A 5-string bass will have the 5th string tuned to B which is 31Hz.


Joined Oct 2, 2009
So what value of cutoff-frequency will generate a natural sound not too low or too high?
That is still a vague question.
What is the definition of "natural sound"?
What is the sound source? radio, TV, smart phone, vinyl record player, computer game station?
Who is the listener, an 18-year old, 55-year or 78-year old person with severe hearing loss?
What kind of electronics and audio equipment are being used to reproduce the sound? What is the size and frequency response of your loudspeakers? Is the loudspeaker a 2"-diameter mp3 bluetooth player or does it have a 15" sub-woofer?

20Hz-20kHz is the accepted range for high quality music.
Most people would not hear the difference if the program were band limited to 100Hz-5kHz.


Joined Oct 2, 2009
You can set the low frequency cutoff to 100Hz and see if that works for you. All of this is subjective.
The upper limit is not infinite. There are many factors that come into play, primarily the frequency response of your circuit and that of the microphone and loudspeaker (not to mention the actual source).

Audioguru again

Joined Oct 21, 2019
Your amplifier in post #13 is biased wrong and is very overloaded.
Its 4.7uF output capacitor passes only high frequency squeaks to the speaker at a very low level.
You need an electronics teacher but I am not a teacher. Goodbye.

Audioguru again

Joined Oct 21, 2019
I am back.
The capacitance of the output coupling capacitor and the impedance it drives determines the low frequency cutoff. Your 4.7uF capacitor driving an 8 ohms speaker from a real low impedance amplifier output cuts 4255Hz and all lower frequencies more. It passes 4255Hz and all higher frequencies.
A 1000uF capacitor from a real amplifier output passes most audio frequencies into an 8 ohms speaker but cuts 20Hz a little and cuts all lower frequencies more.

I wish you can understand that your circuit is extremely overloaded. Its output impedance is 100 ohms then it has trouble driving a load that is 100 ohms or less. When it tries to drive a a load that is 10 ohms then its output power is almost nothing but it wastes a lot of power making heat all the time even when there is no audio signal.

Your circuit is an old class-A heater that rarely used for audio. Most audio amplifiers are class-AB where the class-A operates at idle and at low output power levels then it operates in class-B at high power levels.

Most audio amplifiers use negative feedback (your circuit has none) to reduce distortion, but I added some negative feedback by removing the capacitor parallel to the emitter resistor.

Most audio amplifiers have a push-pull output using 2 transistor emitter-followers that produce an extremely low output impedance that can be seen not to change the output level when loaded or when unloaded. Your amplifier produces almost no output when overloaded.
The output impedance of a Crown class-AB audio amplifier is 0.0053 ohms! It is designed to power an 8 ohm speaker. Its output level does not change when loaded or unloaded.

I properly biased the transistor in your circuit and simulated it with various loads: