Oscillators and VCO Oscillations and Voltage Controlled Oscillators Feedback perspective A a 1 af if af 1 we get infinite gain or oscillations From EE 122 the phase shift oscillator specifically uses series parallel RC network to Make f 1 a and Guarantee exact 0 degree phase shift Timing based oscillations this can be ring oscillator type or charge discharge of C type Transistor level oscillations which we ll do now This discussion while couched in terms of oscillators has relevance to amplifiers as well often in terms of considering how to make the amplifier NOT oscillate Also please refresh your memory about the general feedback expression aka EE113 101B since it is critical when considering impedances as well as gain expressions There is LOTS to say about kinds of oscillators phase shift versus relaxation ring type etc Also it is important to differentiate between sinusoidal oscillations a single tone versus ring and relaxation type oscillators that create triangle or square wave oscillations In lab you will have a chance to consider both a sinusoidal oscillator the so called Colpitts oscillator named after its inventor and a relaxation type oscillator 1 s j Reminder about s plane and poles moving into either LHP or RHP R2 Inverting R1 Gain amp Av R2 R1 Phase shift Network 0 and fo and attenuating by 1 Av Quick reminders from EE122 and EE102 101B about the s plane plot concerning the phase shift oscillator This oscillator has two key kinds of feedback 1 The classical R1 R2 negative feedback which gives a voltage gain as indicated and 2 Feedback around the positive loop where the two square boxes indicate an R C or L R C network that is frequency selective and has a unique frequency where the phase shift is exactly zero At this frequency by design the attenuation of the R C is balanced by the gain of the R1 R2 block such that oscillations are sustained In the s plane we want the poles to be located exactly on the j axis if they are in the Left Half Plane LHP the oscillations will decay away and if in the Right Half Plane RHP the oscillations will grow without bound 2 Timing based Oscillators see Grebene Timer Circuits Schmitt Trigger 555 IC Many others I C C V I x S1 dV dt Tx Control Logic S1 on S2 off Then S2 on S1 off C x is the portion of the total period for which the respective Ix is in control Vcc S2 Now let s turn our attention to the relaxation type oscillator shown schematically here as simply a storage element that is being charged and discharged by two current sources Basically there are many chips that employ this kind of oscillator possibly the one most familiar to you would be the 555 timer chip where all you have to do is select the capacitor and one resistor to create such oscillations The capacitor s governing equation differential form is shown For a constant valued capacitor we can use V and T and for each portion of the charging or discharging of the capacitor T becomes TX Hence the fraction of the period called out as TX is directly proportional to C and V and inversely proportional to IX We can make the wave forms symmetric or asymmetric by changing the values for IX on each portion of the waveform Finally although not written on this slide the frequency of oscillation is given by the INVERSE of the sum of T1 T2 3 Practical Implementations i e the 566 The following slides come from a suplemental handout taken from Alan Grebene s book Bipolar and MOS Analog Integrated Circuits Alan was the lead designer who created the 555 and related products This figure shows schematically the two current sources that charge and discharge the capacitor C1 The Schmitt trigger circuit an old friend from EE122 sets the voltage excursions over which the voltage VO1 will travel I e V VB VA During the charge up period T1 the lower current source is assumed to be off so that the slope of V t is determined only by I1 Once the voltage reaches VB the current source I2 turns on and its value is greater than I1 so that the discharge period T2 is determined by I2 I1 A few more details including the equations to go with the above discussion are shown on the next figure 4 Details about the Timing and Schmitt Trigger T1 T2 VB VA C1 I1 VB VA C1 f I1 1 1 I1 1 T T1 T2 VB VA C1 I2 I2 I1 Simple reminder about the Schmitt trigger It provides a digital logic H and logic L output with a hysteresis in the transitions going L H and H L determined by VA and VB In this figure VH corresponds to VB VA The two fractions of the total period T1 and T2 are determined by the capacitor and current source values as shown in the schematic on the previous page The resulting frequency of oscillation is given by the inverse of the sum of the two times as shown The above discussion has been rather general in terms of allowing the two currents to have arbitrary values The next slide gets down to one specific application where in fact the current is diode steered such that I1 I2 I1 5 Implementation of Current Sources and diode steering of currents This schematic shows an abstracted view of what you will be using in the 566 chip Basically when S1 is OPEN the current I1 can only flow through D2 and charge up C1 When S1 is CLOSED the current mirror a so called Wilson Mirror after it s inventor George Wilson and former Motorola and Tektronix designer pulls the current I1 through D1 and the in turn Q2 forces that same current to flow which then discharges C1 In the process you can easily convince yourself that D2 must be off basically once S1 offers a lower potential path for I1 to flow through the current goes that way and the potential at the cathode side of the diodes drops to that value to be discussed a bit more in class both using this figure and probably the real schematic for the 566 From your perspective as USER of the 566 the control voltage VC sets the current value for I1 and you have the choice of selecting C1 to set the frequency of oscillation Vc can be either dc or V t the former giving a single frequency and the later giving a frequency modulated or FM signal 6 Something really scary Modulation Input Timing Resistor Timing Capacitor I1 Schmitt Trigger Switch I2 S1 This is the schematic of the 566 The key blocks such as the two current sources I1 and I2 are indicated along with the Schmitt Trigger etc In commerical ICs there are LOTS of details that need to be considered to have the chips work over variations in temperature supply voltages etc Hence these are NOT your most basic current sources mirrors