An Audio Compressor/P eak Limiter CircuitThe object of this experiment is to use the vo ltage -controlled amplifier designed in thelast lab to realize an aud io com pressor/ peak limiter circuit. Such a circuit detects when theoutput signal increa ses abov e a p reset limit and reduces the gain of th e circuit to cause th eoutput level to rem ain constan t as the input lev el increases. These circuits are commonlyused in broadcasting and r ecordin g to preven t ov erload of transm itters and reco rders. Th eyare also used for special effects in audio signal processing of voice and music sign als.The difference between a limiter a nd a compress or is primarily in the a ttack and releasetimes of th e circu its. A lim iter redu ces gain quickly when an ov erloa d occurs and then letsthe gain go bac k to its original value quic k ly when the ov erloa d goes a way. For exam p le,if someone drops a microphone in a PA system, the audio pulse generated could blow theloudspeaker s. A limiter can prev ent this. A comp ressor changes gain slo wly comp ared toa limiter . Com pr essors are used to maintain a more constant output level without largec hanges in gain on tran sient peaks. Com b inations of compressor and lim iters are comm on lyused in broadcasting to cause the transmitter to be modu lated continu ously at 100% toincrease the loudness of their signal at the receiver.The basic circuit for the design pr oject is described inhttp://users.ece.gatec h .ed u/~ m leach/papers/limiter.pdfThis circuit uses a p-ch an ne l JFE T . Your design project is to realize this circuit using ann-channel JFET . The circuit is shown in Figure 1. This circuit differs from that in thereference paper in that the JFET is an n-c h ann el device, the diodes are reversed, and thepo larity of t wo power su pply voltages is reversed.1. The first part of this lab is to realize the detector part of the circuit in Figure 1consisting of op amps A2and A3. The circuit is not to be connected to the V CA un tilit is operational. The first step is to assemble the precision rectifier circuit consistingof A2, A3, D1, D2, and resistors R4, R5, R6, R7,andR9.OmitD3, R8,andallofthecircuits to the left of A3. For this part, the left node of resistor R9should co nnect tothe o utput of A3. The specifications are:• The input resista nce is to be 10 kΩ. ThisisequaltoR4kR7. It is suggested that youchoose R4= R7.• The output v oltage from A3is to be a negative going full-wave r ecti fied signal w ith again of unit y. Th at is, the volta ge outp ut of A3is to be vO3= − |vIR4R7|,wherevIR4R7is the voltage app lied to th e righ t node of r esistors R4and R7.• Apply pow er to the circuit. When it is operational, document its performance byrecording typical output waveform s.• The next step is to add resistor R8to the circuit. Calculate its value so that it causestheoutputvoltageofA3to be offset by 1V positive with no input signal. The outputvoltage of A3should now be 1 − |vIR|.• Documen t the operation of th e circuit by recording suitable wav eform s.1Figure 1: Basic circuit d iagram of the compressor/lim iter.22. The next step is to assem b le the curren t mirror circuit consisting of Q1through Q3.DonotconnectthebaseinputtoQ1to the output of A3un til the circuit is operational.Thestepsareasfollows:• With R10=1kΩ and R11= R12=100Ω, assem b le the circuit consisting of Q1throughQ3. Do not ap ply power yet.• Put a diode from the base of Q1to ground. The diode arrow should poin t from baseto ground. This diode is the same as D3in the circu it, except its cath ode shou ld beconnected to ground.• Om ittin g R13and C2, connect the collector of Q3to ground through a 1kΩ resistor.• Connect the function generator in series w ith a 10 kΩ resistor to the base of Q1.• Connect ch ann el A of the oscilloscope to the emitter of Q1and c hannel B to thecollector of Q3.• Apply po wer to the circuit and adjust the function generator to a suitable lev el at1kHz to produce a negativ e going half-w av e rectified signal at the emitter of Q1.Ifthe current mirror is working correctly, y ou should observe the same volta ge at thecollector of Q3. Because the curren t mirror is not perfect, the voltages ma y not beexactly the same. But they should be approximately the same.• Documen t the o peration of the curren t mirror by recording suitable wav eform s.3. When the current mirror circuit is operatio nal, the next step is to connect it to theoutput of the precision rectifier.Thestepsareasfollows:• Connect the output of A3to the base of Q1and reconnect D3and R9as show n Figure1.• Power up the circuit.• Apply a sine wave to the input of the precision rectifier, i.e. to the righ t node s ofresistors R4and R7.• Observe the w av eform at the collector of Q3. You sh ould see a negativ e going vo ltag eeac h time the signal v oltage at the input of the precision rectifier exceeds a peak voltageof 1V (positiv e or negative).• Documen t the operation of th e circuit by recording suitable wav eform s.The circuit will be completed next week b y co nnecting the control c ircuit to the JFET