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Phase Locked Loop - Voltage Controlled Current Source (VCCS)

3.4 The Voltage Controlled Current Source

The Voltage Controlled Current Source is an integral part of the LM-565 Phase Lock Loop. The purpose of this current source is to provide charging and discharging of the timing capacitor, Ctime. This charging and discharging of the capacitor produce a triangle waveform, which is then feed onto the Schmitt, trigger circuit section for voltage level triggering. The Voltage Controlled Current Source can be separated into two main subsections, the upper-half-circuit subsection and the lower-half-circuit subsection which can be seen in Figure 3.4.1.

The division between the two subsections can be made by noting that the current entering the Voltage Controlled Current Source from the source VCC is the same as the current leaving from sum of both IE15 and IE16. Therefore, the logical division is at this point considered the lower-half-circuit and the upper-half-circuit subsection shown in Figure 3.4.2 results.


Figure 3.4.1 Voltage Controlled Current Source Schematic





To find the voltage at VX an equation around the loop containing Q12, Q13 and VCOcontrol can be written using KVL which results in:



VX = VCOCONTROL - VBE12 - VBE13

For NPN Bipolar Junction Transistors (BJT) the base to emitter voltage drop is positive seven-hundred millivolts while the base to emitter voltage of the PNP BJT is negative seven-hundred millivolts. Substituting this into the equation for VX results in VX being approximately equal to VCOCONTROL. Solving for the current through the timing resistor, ICS, the resultant equation is:

ICS = (VCC - VCOCONTROL) / Rtime


Figure 3.4.2 The Voltage Controlled Current Source Upper-Half-Circuit Subsection Division

If a supply voltage of positive six volts is used along with Rtime equal to twenty thousand ohms, VCOcontrol is going to vary between three and a half volts and six volts which causes ICS to vary between one-hundred and twenty-five micro amps to zero amps respectively. The relationship between the voltage VCOCONTROL and the current ICS can be seen in Figure 3.4.3.

At this point, the upper-half-circuit can be simplified to a simple voltage controlled dependant current source supplying the lower-half-circuit with the current ICS. The replacement of the upper-half-circuit and connection to the lower-half-circuit is shown in Figure 3.4.4. This replacement simplifies the analysis of the lower-half-circuit by the reduction to one input node.


Figure 3.4.3 Voltage Controlled Current Source Operation Range


Figure 3.4.4 Lower-Half-Circuit showing Upper-Half Circuit Simplification

With the switch S1 open, also called the charging phase, zero current will flow through D17, Q19, Q20, and Q21. This effectively reduces the circuit to include only ICS, D18, and Ctime, which is shown in Figure 3.4.5. The capacitor Ctime voltage will increase linearly as a function of the capacitor current ICtime. Since no other path exists for the current ICS to flow through except into Ctime, then ICS equals ICtime. The slope of the voltage across the capacitor is determined by the equation:

Slope = ICS / Ctime


Figure 3.4.5 Lower-Half-Circuit with Switch One Open

The variation in VCOCONTROL produced by the phase difference on the input to the Phase Lock Loop causes ICS to vary, causing the slope of the voltage across the timing capacitor to vary. As VCOCONTROL decreases, ICS increases, and the slope increases. This capacitor charging voltage is the first half of the triangle wave that is the input to the Schmitt Trigger.

When S1 is closed by the Schmitt Trigger, the capacitor discharge phase is entered shown in Figure 3.4.6. The closing of S1 allows current to flow through the Emitter Degenerative Resistive Wilson current source made up of Q19, Q20, Q21, R16 and R17. The closing of S1 and the current flowing through the Wilson current source "pulls" the anode of D17 toward negative VCC minus three diode drops and the voltage across R17 or approximately negative three point nine volts, thereby reverse biasing D18. With no current flowing through D18 due to the reverse biased state, all of ICS must flow through D17. Assuming base currents are small and thereby negligible, all of ICS flows through Q20. By operation of a current mirror, the current through Q20 is matched by the current through Q19 and Q21. Since no current can flow through D18 all of the current must come from the capacitor current ICtime, therefore ICS equals ICtime.

The slope of the voltage across the timing capacitor is also determined by the slope equation given above. With S1 switching on and off symmetrically a triangle waveform is generated across the timing capacitor. By varying VCOCONTROL, the magnitude of the slope of the triangle waveform varies. As VCOCONTROL increases, the slope decreases and ICS decreases.




Figure 3.4.6 Lower-Half-Circuit with Switch One Closed

The control and predictability of ICS is accomplished using some advanced techniques in semiconductor construction. In the upper-half-circuit, a triple collector PNP BJT is used for current matching. In most BJTs the collector contact is a solid ring around the emitter contact. When the collector contact is equally divided into two, three or four contacts, the current from each section is exactly matched which is essential for the correct operation of the upper-half-circuit. The topology and schematic symbol is shown in Figure 3.4.7.


Figure 3.4.7 Triple Collector PNP BJT Topology and Schematic Symbol

The upper-half-circuit shown in Figure 3.4.8 is the Signetics schematic diagram. A simplified form is shown in Figure 3.4.9 where the triple collector PNP BJT is shown as three separate BJTs. ICS is separated into exactly quarters by using the triple collector BJT. Thus, the current through Q15 exactly matches the current through Q16 and the sum of the two currents equals ICS.



Figure 3.4.8 Upper-Half-Circuit Schematic Diagram


Figure 3.4.9 Upper-Half-Circuit Simplified Schematic Diagram

Figure 3.4.10 shows the PSPICE simulation of the Voltage Controlled Current Source. The triangle waveform is the voltage across the capacitor which is provided to the Schmitt Trigger. The second waveform is the voltage across the diode connected transistor Q18 showing the forward and reverse bias conditions. The positive voltage is the capacitor charging phase and the negative voltage is the capacitor discharge phase.


Figure 3.4.10 PSPICE Simulation of the Voltage Controlled Current Source







Electrical Engineering lab key words: VCCS, voltage controlled current source, PLL, phased locked loop, quality, clean current, load, resistor, resistance, variable, buffer circuit, clipping, amplifier, amplification, max load, temperature, voltmeter, ammeter, gain and depend sources, Schmitt trigger, timing capacitor, KVL, capacitor discharge, BJT, ICS, VCO, relationship, node, control signal, diode, emitter, collector, Wilson current source, anode, PNP, NPN, IC, BJT matched pairs, Pspice.

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