# BJT Inverter

OBJECTIVE:

The objective of this webpage is to further demonstrate the different modes of operation of a typical BJT Inverter. Through simulation tools such as PSpice and MoHAT as well as through the analytical method, we will find the critical voltages of the BJT Inverter (VOH, VIH, VOL, VIL).

A TYPICAL BJT INVERTER: Figure 1: Schematic of typical BJT Inverter

MODES OF OPERATION:

 BE JUNCTION OFF BE JUNCTION ON BC JUNCTION OFF Cut-off Forward Active BC JUNCTION ON Reverse Active Saturation

CRITICAL VALUES:

The critical values of a typical BJT Inverter cab be retrieved from it's Voltage Transfer Characteristic graph (VTC) like in Figure 2. Figure 2: VTC of BJT Inverter

Following the VTC, we can obtain the critical values of the inverter:

• VOH = Vcc
• VIH = VBE(sat) + (Ib * Rb) ---> simplified ---> VBE(sat) + [ { (Vcc - VCE(sat) ) / (Bf * Rc) } * Rb ]
• VOL = VCE(sat)
• VM = [ { VBE(on) + (Rb/Bf*Rc)*Vcc } / { 1 + (Rb/Bf*Rc) } ] ---> VM is where Vin = Vout in the Forward Active mode of operation
• NMH = VOH - VIH
• NML = VIL - VOL

given the standard values of the various voltages for the inverter:

• VBE(eoc) = 0.6V
• VBE(on) = 0.7V
• VBE(sat) = 0.8V
• VCE(sat) = 0.1V

DESIGN/IMPLEMENTATION:

For this typical BJT Inverter:

• Vcc = 5.0V
• Rb = 10 kOhm
• Rc = 1kOhm
• Bf = 50 Figure 3: Design Example of BJT Inverter

Solving analytically for this inverter's critical values:

• VOH = Vcc ---> VOH = 5V
• VIH = VBE(sat) + [ { (Vcc - VCE(sat) ) / (Bf * Rc) } * Rb ] ---> VIH = 1.78 V
• VOL = VCE(sat) ---> VOL = 0.1 V
• VIL = VBE(eoc) ---> VIL = 0.6 V
• VM = [ { VBE(on) + (Rb/Bf*Rc)*Vcc } / { 1 + (Rb/Bf*Rc) } ] ---> VM = 1.417 V
• NMH = VOH - VIH ---> NMH = 3.22 V
• NML = VIL - VOL ---> NML = 0.5 V

USING PSPICE CIRCUIT MODEL:

* EE 307 BJT Inverter
* Archana Datla, Michael Casanova
* BJT Inverter Design, MoHAT Project #1

Vin 2 0
Vdd 1 0 5.0

RB 2 4 10k
RC 1 3 1k
Q1 3 4 0 BJT1

.MODEL BJT1 NPN (BF=50 CJE=0.6p CJC=0.58p CJS=2.8p
+VJE=0.715)

.DC Vin 0.0 5.0 0.005
.PRINT DC V(2) V(1) V(4) V(3)
.PRINT DC I(RB) I(RC) I(Vdd) I(Vin) IB(Q1) IC(Q1) IE(Q1)

.PROBE
.END

PSPice OUTPUT: Figure 4: PSpice output of inverter's VTC Figure 5: PSpice output for inverter's VTC--Zoom in of D(Vout) vs. Vin to show VIL, VIH

From PSPice's VTC, the results of the critical values are very close to the results obtained analytically. In Figure 4, the VTC was plotted by tracing Vout vs. Vin to obtain VOH, VIH, VOL, VIL, VM. In Figure 5, d(Vout) vs. Vin was traced to obtain more accurate values of VIL and VIH. With PSpice, the critical voltages are:

• VOH = 5V
• VIH = 1.80V
• VOL = 0.1V
• VIL = 0.69V
• VM = 1.5V

USING MoHAT SIMULATION TOOL:

Now by using the MoHAT simulation tool, we will find the same critical voltages of the BJT inverter. By viewing the given VTC as well as when the inverter changes modes. The simulation tool tells you when the inverter changes modes of operation by color-coding its different states (color code is provided by the given key).

At Voltage Output High: Figure 6: Vout = VOH, Mode of Operation: Cut-off (Vin < VIL)

At Voltage Input Low: Figure 7: Vin = VIL, Mode of Operation: Forward Active (VIL < Vin < VIH)

At Threshold Voltage: Figure 8: Vin = VM = Vout, Mode of Operation: Forward Active (VL < Vin < VIH)

At Voltage Input High: Figure 9: Vin = VIH, Mode of Operation: Saturation (Vin > VIH)

At Voltage Ouput Low: Figure 10: Vout = VOL, Mode of Operation: Saturation (Vin > VIH)

CONCLUSION:

By using MoHAT simulation tool, we can see that we obtained the same results as solving the critical voltages either analytically or with PSpice. By testing various inputs at various modes of operation, our circuit follows through with the theoritical as well as a simulated application.

VOH : (Cut-off)

• Analytic Method: VOH = 5.0V
• PSpice Tool: VOH = 5.0V
• MoHAT Simulation Tool: VOH = V(3) = 5.0V

VIL : (Forward Active -- Edge of Conduction)

• Analytic Method: VIL = 0.6V
• PSpice Tool: VIL = 0.69V
• MoHAT Simulation Tool: VIL = 0.675V

VM: (Forward Active)

• Analytic Method: VM = 1.417V
• PSpice Tool: VM = 1.50V
• MoHAT Simulation Tool: VM = 1.50V

VIH: (Saturation -- Edge of Saturation)

• Analytic Method: VIH = 1.780V
• PSpice Tool: VIH = 1.80V
• MoHAT Simulation Tool: VIH = 1.90V

VOL: (Saturation)

• Analytic Method: VOL = 0.1V
• PSpice Tool: VOL = 0.1V
• MoHAT Simulation Tool: VOL = 0.1099V
• Created by: Archana Datla, Michael Casanova
• EE 307-01, Fall 2003
• Prof: Dr. D. Braun