Aim:

To design and simulate a RC Coupled Amplifier circuit.

Design & Theory

A transistor amplifier is designed to work in the active region. A single stage amplifier is designed using voltage divider bias. In a voltage divider bias the base voltage, V_B is given by

First we bias the circuit in the active region with dc conditions.

i.e. Current through R1, I_R1 is a large current of the order of 20 or 30 times that of I_B.

Current through R2, I_R2 is given by

We select V_CC as 10V.

Minimum Voltage Gain required = 100

Let I_CQ = 2mA – ref. Manufacturer’s Datasheet

Applying Kirchoff’s Voltage Law for the output side of amplifier

For a fluctuation in I_R1 and I_R2 there will be small change in I_B. For example as mentioned (ref. (5)), if I_R1=21I_B and a 5% change in I_R1 occurs there will be only change in I_B. Therefore the circuit will be stable against small changes in R₁ and R₂ due to temperature or tolerance.

Thus

And

=0.7+0.1V_CC

From (8)

From (9)

Modifications required from the dc circuit are

1. Coupling capacitors (C_in, C_out)
2. Bypass Capacitor C_E.

The function of the coupling capacitor is to isolate the amplifier input circuit from the source. Since capacitor blocks dc it does not allow the dc components from the input circuit to get to the base of transistor and to change the Q point.

Additional requirement if C_in is that it must pass the input ac signal unattenuated. Hence the lower cut-off frequency (LCF) of the amplifier is determined by the coupling capacitor. The coupling capacitor Cin along with resistance combinations shown in fig forms a high pass filter whose cut-off frequency; .

The LCF due to Cin;

At mid or high frequencies capacitor almost acts as a short circuit.

Then V_i is related to V_S by;

The effect of C_out on LCF is given by; where .

C_E is designed such that impedance of the capacitor must be less than of R_E across which it is connected at LCF. The function of C_E is to bypass the emitter resistor during ac operation other wise during amplification with increase or decrease in current, the drop across R_E will increase or decrease and hence V_BE decreases or increases opposing the change in current. So the function of R_E is limited to dc where the capacitor will be open and R_E will stabilize the circuit. In the case of ac the capacitor will take over current.

Form (15)

h_ie = 1kW (datasheet)

Let f_Lin = 100Hz

where

For pn junction current I is related to voltage V by equation

where h =1 for Ge and 2 for Si.

where T_C is Room temperature in ° C

T_K = 25+273=298K

V_T – Voltage equivalent of temperature

I_S – Reverse saturation current

Now

Let f_LE = 100 Hz

To create a new part select EDWin 2000 Main -> Library -> Part Editor -> File -> New Part.

For e.g.: A transistor may be created in the following manner. Type in the information in the necessary fields (Prefix, Description, Type, Manufacturer etc.). The symbol name may be given as NPN. It may be created manually or using Wizard. The symbol may be drawn using ‘Create Graphic Item’ function tool and its option tools. A filled bar is drawn using option tools rectangle and filled item. The filled triangle drawn using the option tools triangle and filled item represents the emitter and the direction of electron flow. Place the entries using ‘Create Entry’ option tool of ‘Create Graphic Item’ function tool. Then entries may be named as Emitter, Base and Collector using function tool ‘Edit Entry Attributes’. Now place the Component Name and Description appropriately.

In order to assign Simulation Names follow these steps. Select the ‘Properties’ function tool and its option tool ‘Symbol Properties’. Now select the desired simulation function from the list. Using ‘Edit Entry Attributes’ function tool click on each entry and assign proper Simulation Name to each pin. Save the Symbol/ Part either in user defined libraries and add them to the search sequence or in the Project Library itself.

To place the components select Schematic Editor -> Components. The components may be loaded in any of the following ways.

1. Add Component: Type in the name of the component to be added to the project. On clicking accept an image of the component gets tagged to the cursor. Click anywhere on the workspace to place it. On each click on the workspace the component gets repeated until the Esc key is pressed.
2. Library Explorer: Use Drag-drop or Send-to feature to load the component.
3. Library Browser: Use Drag-drop or Send-to feature to load the component.

Place these loaded components in the appropriate position using the ‘Relocate’ function tool and its option tool(s).

To wire the components select Schematic Editor -> Wires & Buses. The ‘Route Wire’ function tool and its appropriate option tool(s) may be used for this purpose. Enable option tool ‘Pin-to-pin routing’ for starting and ending wiring from a component entry. Otherwise the routing may start very near to component entry which causes netlist problem. Prior to this ‘Instant Net Name’ and ‘Instant Wire Label’ may be enabled in ‘Preferences’ menu, so that the net name can be given as required and labels for the same can be placed without using any additional tool(s). Enable the option ‘Nodes’ in View menu, so that the proper assignment of net to the component entries can be ensured

The instance parameter (Component value) may be assigned either in Schematic Editor or in Simulators. In Schematic editor this is performed using the function tool ‘Add/Change Component text’ and its option tool ‘Add / Change Value’. It is mandatory to use its option tool for this purpose and the simulators will ignore any values assigned without using this option tool.

The purpose of preprocessing is to establish the degree to which the circuit is ready for simulation. The results of preprocessing are displayed in the "EDWin 2000 Info " window. This should be done in both the simulators prior to performing any simulation.

Simulation is performed in both Mixed Mode and EDSpice Simulator. In Mixed Mode simulator we are performing Transient Analysis, DC Sweep and AC Sweep Analysis.

In Transient Analysis, we perform the simulation of the circuit and analyze the output voltage with respect to time. For this maximum simulation time and the time limit are set in the corresponding parameters window. Then simulation is performed with ‘Display Waveform’ option enabled. The output of the amplifier is viewed in the Waveform viewer. The user also has the provision to simulate with print and plot outputs.

In this type of analysis, usually temperature is the sweeping parameter. The user has the provision of sweeping parameters of any two components at a time.

AC Sweep Analysis is performed for analyzing the frequency response of the circuit. It is required to run AC Sweep Analysis after DC Sweep Analysis, so that the circuit operating parameters would have been stabilized and set. The process is similar to Transient analysis with difference in the parameter setup.

Assign the parameters for each and every component using the function tool ‘Set parameters/ models’ and its option tool ‘Set parameter’. Place the test points appropriately so that the voltage or current of that particular node or net has to be displayed. Now place the waveform markers at the node or net where the waveform has to be observed. Preprocessing of the circuit must be carried out before simulating it.

Set all the necessary values in the waveform viewer as set in a CRO.

The output waveform for various analyses is shown below.