Current collectors

The collector current in this case consists of two components... 

Activated carbons possess sufficient volumetric conductivity for electrolyte/collector current interchange. However, contact resistance between carbon particles in the electrode limits charge/discharge currents of the porous volumetric system and therefore EC s power capability. 

Log of the limiting steady-state generator/collector current of an interdigitated array of microelectrodes coated with poly(I) vs. 1/T. 

The of the cell was measured as a function of the collector current with different collector supply voltages (Figure 6.29). For = 20V, peaks at about 1.5 GHz, whereas for V = 30V, fj. peaks at about 1.3 GHz. This reduction in f. with collector voltage is expected as the electron transit delay in the collector depletion width increases with increasing collector bias. The sudden drop in at high current for V c = 30V is attributed to a hot spot on the device. 

Figure 6.29 Measurement of fj as a function of collector current and the collector voltage.

For a BJT, the Bias Point Detail gives the collector current, the collector-emitter voltage, and some small-signal parameters for the BJT at the bias point. For a jFET, the Bias Point Detail gives the drain current, the drain-source voltage, and some small-signal model parameters at the bias point. The results of the Bias Point Detail are contained in the output file. We will illustrate the Bias Point Detail analysis with the circuit below ... 

In this section we will investigate how the DC current gain (Hfe) of a bipolar junction transistor varies with DC bias collector current Icq, DC bias collector-emitter voltage Vceq, and temperature. We will use the basic circuit shown below for all simulations ... 

Note that the x-axis is the base current. This is a plot of Hfe versus IB. We need to change the x-axis to collector current. From the Probe menus, select Plot and then Axis Settings ... 

We must change the x-axis variable to collector current. Click the Axis Variable button ... 

We see that the 2N3904 BJT has a maximum DC current gain at a collector current of about 10 mA. Use the cursors and label the maximum value of current gain ... 

EXERCISE 141 For the MJE3055T NPN power transistor, find the collector current where Hfe is maximum. Specify the maximum value of Hfe and the collector current where it occurs. Let Vce be constant at 5 V. 

Since this is a power transistor, it can handle much higher collector currents. Sweep IB from 100 pA to 100 mA ... 

The constant current source keeps the emitter current constant at 10 mA. Since the collector current is approximately equal to the emitter current, the constant current source keeps the collector current approximately constant at 10 mA. Since the base is grounded, the emitter voltage is a diode drop below ground ... 

There are two ways to look at this circuit One is that this is a self-biasing circuit for a BJT amplifier. If this were an amplifier, the load would most likely be a resistor. (The term "self-bias means that the goal of the circuit is to make the collector current independent of device parameters such as Hfe and Vbe.). The second way to look at the circuit is that as far as the load is concerned, Ql, Rl, R2, and R3 form a current source that is, the current through the load is determined by Ql. Rl, R2, and R3. If this circuit were designed as a current source for the load, we could view the circuit as ... 

No matter what the application, the circuit is designed to make the collector current independent of VBE and Hfe-We will assume that the resistors used have no tolerance and have their exact values specified in the circuit. To see how the tolerance of resistors affects the collector current, see Section 9.C. From previous sections we know that the resistance of a resistor, and Hfe and VBe of a BJT, are affected by temperature. The question is, if the temperature changes, how much does the collector current change ... 

We will perform two analyses on the circuit. The first will be a plot of collector current versus temperature with ideal resistors. The resistors will have no temperature dependence. The only temperature-dependent device will be the BJT. The second simulation will use temperature dependence for resistors and the BJT. For simulation, we will use the circuit below ... 

First, we will use PSpice to find the collector current without temperature dependence using a bias point analysis and display the collector current on the schematic. See Section 3.A for the procedure to display currents on the schematic ... 

We have specified a linear sweep from -25°C to 125°C with 1-degree increments. Click the OK button to the schematic. Simulate the circuit and display the results with Probe. Add the trace IC (Q1) to plot the collector current. Use the cursors to label the end points of the range. 

We see that, over the temperature range, the collector current varies from -889 pA to -1.317 mA. It is not very independent of temperature. Remember that this variation is without resistor temperature dependence. We could make the circuit more temperature independent by choosing smaller values for R2 and R3. However, this will consume more power. 

We see that with both resistor and BJT temperature dependence, the collector varies from -894 pA to -1.304 mA. In our simulation with only BJT temperature dependence, the collector current ranged from -889 pA to -1.317 mA. Our conclusion is that most of the temperature dependence in this circuit is due to the BJT. 

We see that both nodes labeled Vref are at a voltage of 10.01 volts. The negative feedback attempts to keep these two voltages the same. The emitter current is 1.001 mA and the collector current is 997 XA. Since Ic = [P/(P+1)]Ie, we expect the collector current to be slightly less than 1 mA, in this case 3 pA. 

We see that the voltage is, for all practical purposes, constant at 10 V. Next we will plot the collector current of the BJT ... 

Even with resistor temperature dependence, the collector current changes by only about 13 pA over the entire temperature range. This circuit is a very temperature-independent constant current source, as long as the temperatures of R1 and R2 are the same and they have similar temperature coefficients. ... 

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