EE 307 Section 2 MoHAT Project
Philippe Gonzaga (pgonzaga@calpoly.edu)
The purpose of this page is to guide those interested in the fundamental operations of a BJT inverter. You will find below, graphical figures of the different modes of operation for this type of inverter and modes that occur in its use. The graphics depicted are taken from an online graphical circuit simulator called MoHAT, available at www.mohat.net.
The BJT inverter displayed below contains a DC voltage source input, varying from 0V to 5V, connected to a 10KO resistor set on the base of the BJT. A 5V Voltage source connected to a 1O resistor is on the collector of the BJT, while the emitter of the BJT is connected to the ground. The voltage output is located in between the collector and the 1kO resistor at voltage node V(4).
PSpice, a circuit simulation program, was used to analyze the characteristics
of the BJT inverter with the given properties. The information received from
the PSpice analysis gave the various critical voltage points of the BJT inverter,
the said points being the output voltage high, Voh, the input voltage low,
Vil, the input voltage high, Vih, the output voltage low, Vol and the voltage
where the output and input are equal, Vm. The information output from PSpice
was taken and then inputted into MoHat for graphical analysis. MoHat was
then used to determine the BJT's modes of operation at varying input voltages.
Definitions:
VOH: The maximum voltage that occurs on the output.
VOL: The minimum voltage that occurs on the output.
VIL: The voltage that occurs when the slope of the VTC equals 1 and the BJT
is on the Edge of Conduction.
VIH: The voltage when the slope of the VTC equals 1 and the BJT is on the
Edge of Saturation. VM: Voltage point on the VTC where the input voltage is
equal to the output voltage.
Voltage Transfer Characteristics(VTC): A graph of the input voltage versus
the output voltage.
Cutoff: BJT mode where the input voltage is smaller than cutin voltage.
Forward Active: BJT mode where the input voltage is larger than the cutin
voltage and the collector voltage is larger than 0.1V.
Saturation: BJT mode in which the voltage input is higher than the cutin voltage
and the collector voltage is equal to 0.1V.
Critical point and mode of operation 
VTC & Notes 
Circuit Diagram 

BJT in cut off mode VIN = 0V; VOUT = 5.0V; VIN is below cutin voltage, BJT in cutoff mode so it is off. 

VIL; BJT in Forward active mode, near the edge of conduction 
VIN = 0.551V; VOUT = 4.9V; VIN just passed the edge of conduction and has entered Forward Active Mode.  

VIN = VOUT = 0.757 V; In order to find the exact VM smaller increments in the PSpice analysis were used. The BJT is in Forward Active Mode.  
BJT in saturation, 
VIN = 0.7750V; VOUT = 0.098V; The VOUT just went under 0.1V where it goes from forward active to saturation.  
Vol; in saturation  Vin = Voh = 5V; VOUT = 0.0195V; BJT stays in saturation. And this is the lowest the VOUT will go. 
Noise Margins:
The noise margins of a circuit give the amount of sensitivity the circuit has in input fluctuations. The two noise margins define where the logic high, NMh, and logic low, NMl, occur. NMH is calculated by: NMH = VOH  VIH. NML is calculated by: NML = VIL  VOL.
The BJT circuit defined here has the Noise margins of:
NMH: 5V  0.775V = 4.225V
NML: 0.551V – 0.0195V = 0.5315V
PSpice File used:
* BJT Inverter
* EE 307 Sec 02
* Braun
* Philippe Gonzaga
Vdd 1 0 DC 5V
Vin 2 0
RB 2 3 10K
RC 1 4 1K
* C B E (Collector Base Emitter)
Q1 4 3 0 BJTINV
.MODEL BJTINV NPN(BF=80 CJE=0.6p CJC=0.58p CJS=2.8p
+VJE=0.715)
.DC Vin 0 5V 0.001
.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:
VTC with markings of Voh, Vil, Vm, Vih, Vol
Varying various components and characteristics of the BJT inverter can adversely change the voltage transfer characteristics. The resistor values of RB and RC change both the slope of the VTC and the ending VOL. Increasing RB’s value decreases the slope of the VTC and increases the VOL, whereas decreasing RB increases the slope of the VTC and decreases VOL. Increasing RC does the opposite of increasing RB, it increases the slope and lowers the VOL, decreasing RC does the opposite. Decreasing the ßF alters the slope of the VTC only by decreasing it, while an increase makes the VTC drop faster.
Conclusion:
In conclusion the BJT inverter is a very stable inverter and adjusting its
VTC is fairly easy. By simply changing the resistor values, one can attain
a desired VTC and obtain desired critical voltages. The BJT’s modes
of operation are also easily found due to the fact that it can only be in
three modes, cutoff, forward active and saturated. These two factors of
easy modifiability and easy steps to find its mode of operation make the
BJT a very useful inverter.