EE 462: Laboratory Assignment 6 Biasing of Transistors: N- channel MOSFET

Transcription

1 EE 46: Laboratory Assignment 6 Biasing of Transistors: N channel MOFET by r. A.V. adun and r... onohue (10//03 eartment of Elecical and Comuter Engineering University of entucky Lexington, Y Laboratory # 6 Prelab due at lab sessions October 14, 15, and 16. Lab due at lab sessions October 1,, and 3.. nsuctional Objectives Analyze the metal oxide semiconductor (MO field effect ansistor (FET (MOFET using a C load line esign a circuit to set a C oerating oint for a MOFET Measure the oerating oints in a C biased FET circuit ee Horenstein 5., 7.3.1, and Background Transistors are nonlinear devices, but over certain oerating regions they can be aroximated with linear models. To ensure a ansistor oerates in its linear region, a C level is added to its inut signal. The design of this C level is referred to as biasing the ansistor, and the C values of the ansistor's currents and voltages are referred to as the ansistor s C oerating oint, its bias oint, or its quiescent oerating oint. Once a ansistor is biased in its linear region, currents and voltages in a neighborhood around the bias oint will vary linearly (aroximately. t is assumed that the variation of the ansistor's inut signal and the ansistor's currents and voltages are small enough that they do not move the system into nonlinear regions of oeration (iode region or cutoff. The simlest common source MOFET amlifier biasing scheme is shown in Fig. 1. ince only the C oerating oint is of interest right now, the time varying art of the inut signal is omitted. The actual signal would be an AC signal added to the V (ate to round C signal. The ansfer characteristic (outut as a function of the inut for this circuit can be derived. Aly VL in Fig. 1 to obtain: V (1 out V,

2 then using a relationshi between and the gate voltage V when the MOFET is in the saturated (forward active region, Eq. (1 becomes: V ( out V V V where uses, ( (Horrenstein uses and ice and V is the threshold voltage between the cutoff and iode oeration region. V out V V Fig. 1 Basic common source amlifier biasing. The oerating oints of V and V out are the C or quiescent values at the inut and outut, resectively. deally, for a given V, V out should not vary if the temerature varies or if different ansistors of the same tye are used. Unfortunately the MOFET's ansconductance arameter cannot be conolled well during manufacturing. n addition, also varies with temerature. For examle, the N7000 MOFET ansistor s can vary by more than 3. to 1. Note that is not directly secified in the data sheet but rather the ansconductance gm is secified. The relationshi between the MOFET s ansconductance gm and will be addressed in future labs. ince varies significantly, a circuit biased correctly for one ansistor may not be biased correctly for another ansistor of the same manufacture and art number. Therefore, a more robust biasing scheme than the one shown in Fig. 1 is needed, such that the MOFET's quiescent oerating oint is less sensitive to changes in. nsensitivity of the MOFET's quiescent oerating oint can be achieved by adding a resistor into the source branch of the circuit as shown in the Fig.. An analysis this new circuit, similar to before, results in the following equations: V V (3

3 ( V V ( V V (4 (5 0 V V V out Fig. Basic common source amlifier with reduced sensitivity. Note that when goes to zero (multily by to: (6 and let go to zero, Eq. (5 simlifies which is the same relationshi for the circuit in Fig. 1. (Why should this be exected? One the other hand, if the is large (relative to, then Eq. (5 is aroximated by: (7 0 Notice that in Eq. (5, only aeared in one term, whose effect was minimized (relative to the other terms by a large value. The quadratic in Eq. (7 is a erfect square and can be rewritten as:

4 (8 0 or Thus if is large, is ractically indeendent of as desired. The general solution for valid for any is: (9 1 1 From Eq. (9, the aroximation of Eq. (8 can also be obtained by letting s get large. This circuit of Fig. is still not as efficient as it can be because it requires two ower sulies: one for V and another for V. The circuit in Fig. 3 remedies this. The Thévenin equivalent for the circuit consisting of V, 1, and in Fig. 3 gives the biasing circuit in Fig. where V V th and th. With these Thévenin equivalents substituted into the circuit, the circuit is identical to the circuit in Fig. with the excetion that the bias voltage V is now deendent on V. The V voltage is now conolled by the roer choice of 1 and. This eliminates the need for a searate ower suly to conol V. The gate current into the MOFET is zero so the gate voltage is thus determined by only V and the 1, voltage divider. 1 V V out Fig. 3 Basic common source amlifier biasing with reduced sensitivity and emloying a single C voltage. The C Oerating oint of this circuit is stable for two rimary reasons:

5 The gate voltage is determined by only the voltage divider 1 and, since insignificant current flows into the ansistor gate, and is therefore indeendent of the ansistor arameters (esecially. The source resistor stabilizes the C oerating oint through negative feedback. f increases for any reason, such as temerature change, the subsequent rise in source current will increase the voltage dro across, thereby increasing V and thus decreasing V (since the gate voltage is a constant. The decrease in V will counteract the attemted increase in.. PreLaboratory Exercise The MOFET you will be using in the lab is the ZVN3306A with a secified at A / V (or for the older kits N7000 with a 0.5 A / V (it can be as small as a third of this value and the nominal value of the threshold voltage is V 1.8 (or V.1 V. You are to use 1000 Ω, 470 Ω, and V 10 V. Note that this relab is easier if you use a Matlab or Mathcad or Male, etc. 1. erive the equation for the C load line for the MOFET circuit in Fig. 3 (with 1, and the source resistor used for added circuit stability in terms of the variables V,, and (irchhoff s voltage law.. Comute the maximum ossible drain current and the corresonding gate to source voltage when the MOFET is in the saturated region. What is the smallest drain to source voltage to be in the saturated region for this maximum drain current condition? 3. etermine the quiescent oerating oint Q and V Q such that it is at the midoint of the C load line in the saturated region (to ensure maximum symmeical outut voltage swing. What is the voltage across and at this design oint? 4. Plot the C load line suerosed with the characteristic curves ( versus V of the Nchannel MOFET. 5. What is the gate to source voltage at the design oint (quiescent oerating oint and the gate to ground voltage (voltage across? 6. etermine the value of 1 and, which will maintain the ansistor at that oerating oint. Make 1 Q / ecrease to one half and then 1/3 its design value and determine the change in the oerating oint ( Q (using Eq. (9 that occurs while keeing the same resistor values (the same V V in Eq. (9. 8. Create a PCE model for the circuit in Fig. 3 with the values given above and your comuted 1 and. Use the MOFET ZVN3306A /N7000 model and change the P and V values to values secified here. 9. o a PCE oerating oint analysis to determine the voltage from the drain to the ground and the drain current. 10. o a PCE oerating oint analysis to determine voltage from the drain to ground and the drain current when has been decreased to one half of its design value. V. Laboratory Exercise

6 1. Measure the ZVN3306A /N7000 MOFET s drain characteristic curves. etermine the MOFET s and threshold value?. Consuct the MOFET common source amlifier (Fig. 3 using the Prelab values for 1,, and V. 3. Measure the circuit's quiescent oerating oint by measuring V Q, the exact value of and the voltage across or in order to calculate Q. Measure the quiescent oerating oint using your multimeter on the C settings and using your oscilloscoe on the C settings. (iscussion: Comare results between the oscilloscoe and multimeter. 4. Aly a 1V rms, 10 Hz, sine wave to the inut through a 10 µf caacitor (ee Fig. 4, observe the inut, and outut signals on the oscilloscoe. Make sure the caacitors olarity is correct. etermine the voltage gain v ˆ V out out( t VoutQ. ain vˆ in V V Measure the inut and the outut voltages using your multimeter on the C and AC settings and using your oscilloscoe on the C and AC settings. (iscussion: Comare results between the settings. Why is V inq 0 V, what would haen if it had a C offset? 5. ncrease V to 15 V and measure the circuit s quiescent oerating oint (only needs to be measured one way. (iscussion: How did the quiescent oerating oint change and how did the circuits load line change? id the circuit's quiescent oerating oint stay at the midoint of the load line? id the voltage gain change? in inq C 1 V V in Vout Fig. 4. Basic common source amlifier biasing with reduced sensitivity and emloying a single C voltage with an AC inut voltage.