Topic 4  Operational Amplifiers

Amplifier Basics

Analogue electronics involves a continuously varying signal that can have any value, positive or negative; the only limit is the value of the power supply.  The graphs below compares analogue and digital signals.

Notice that digital signals do not fall below 0 V.  The difference between analogue and digital signals is:

• Analogue signal can have any value between +Vs and –Vs 

• Digital signal is 0 or 1 and does not fall below 0 V.

Analogue systems are widely used in:

• Radio broadcasting

• Telephony

• High Fidelity music reproduction.

• Television.

In effect, any use where we need to see or hear things.  Our eyes and ears respond to analogue signals, not digital.

Question 1.  What is the difference between analogue and digital signals?  ANSWER

The output voltage from an input device such as a microphone, or tape head in a cassette recorder is very small, in the order of millivolts.  The loudspeaker takes a voltage of 20 V.  Therefore there needs to be a way of boosting the voltage to a level at which it can be used.  A transformer is no good, because, as the voltage is increased, the current is reduced.  We have to find a way of increasing the current as well as the voltage.  This is done using an amplifier.

The extent to which the amplifier increases the voltage, current, or power is called the gain.  It is the ratio of the output voltage (or current, or power) to the input voltage (or current, or power):

•                                     Gain = Output voltage

•                                                    Input voltage

  In this section of work we will consider the voltage gain.

Let us now look at what happens when we apply a sinusoidal voltage to the input of an amplifier and see its effect on the output voltage.

Notice that the phase of the wave is changed by 180^o (p radians).  This means that the output wave is upside down compared to the input wave. 

  Question 2 Are visual signals digital or analogue?  ANSWER

If the average level of the input signal too large, we will get distortion.

Notice that the peaks and troughs of the waves are cut off.  This is called clipping and results in a noticeable distortion.  Bad clipping can make an audio signal at best unpleasant to listen to, at worst unintelligible.

Bandwidth

The bandwidth of the amplifier is the range of frequency at which the power is at least half the maximum power.

If a loudspeaker provides a constant load, we can say that a voltage rise of root 2 times gives a doubling of power.  This is because the current will also go up root 2 times.  Therefore we can define the bandwidth as:

 Question 3 An amplifier has a gain of 200.  The input voltage is 75 mV, what is the output voltage?  ANSWER                                                                                                          

Feedback

Feedback is a term that electronic engineers use in which a fraction of the output is taken back to the input.  Negative feedback is widely used in amplifier circuits as it reduces the gain.  It also makes the amplifier more stable.  Amplifiers without negative feedback tend to be rather unstable.  This can arise due to:

• Temperature differences

• Stray inductance and capacitance effects

• Noise within the components or from poor soldering.

• Fluctuations from the power supply.

  The effect of an unstable amplifier is that the output becomes distorted in an unpredictable and random way.

Positive Feedback

The microphone picks up some sound and this goes with the input signal to be amplified.  This makes the sound louder, so the input to the microphone gets larger, which gives a larger output…  The result of this is a feedback loop or howl round.  Whatever you call it, it sounds the same, an ear shattering boom or screech.  Positive feedback is used constructively in circuits such as the Schmitt Trigger or oscillator circuits.

Negative Feedback

This is what happens:

• The output is fed into the inverting input,
• The phase will be changed by 180^o.  
• This will reduce the output.
• This is added to the input wave, reducing the size of the input.

The graph shows this:

  Question 4 How does negative feedback reduces the input to an amplifier? Think about the phase changes.  ANSWER

Click HERE to move on to the operational amplifier.

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