RC Coupled Amplifier
To design and simulate a RC Coupled Amplifier circuit.
EDWin Components Used
Number of components required
|VGEN||SMB_VGEN||Ac voltage source||1|
|VDC||SMB_VDC||Dc voltage source||1|
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, VB is given by
First we bias the circuit in the active region with dc conditions.
i.e. Current through R1, IR1 is a large current of the order of 20 or 30 times that of IB.
Current through R2, IR2 is given by
We select VCC as 10V.
Minimum Voltage Gain required = 100
Let ICQ = 2mA ref. Manufacturers Datasheet
Applying Kirchoffs Voltage Law for the output side of amplifier
Design of R1 & R2
For a fluctuation in IR1 and IR2 there will be small change in IB. For example as mentioned (ref. (5)), if IR1=21IB and a 5% change in IR1 occurs there will be only change in IB. Therefore the circuit will be stable against small changes in R1 and R2 due to temperature or tolerance.
Modifications required from the dc circuit are
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 Cin 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 Vi is related to VS by;
Effect of Cout
The effect of Cout on LCF is given by; where .
Effect of CE
CE is designed such that impedance of the capacitor must be less than of RE across which it is connected at LCF. The function of CE is to bypass the emitter resistor during ac operation other wise during amplification with increase or decrease in current, the drop across RE will increase or decrease and hence VBE decreases or increases opposing the change in current. So the function of RE is limited to dc where the capacitor will be open and RE will stabilize the circuit. In the case of ac the capacitor will take over current.
hie = 1kW (datasheet)
Let fLin = 100Hz
For pn junction current I is related to voltage V by equation
where h =1 for Ge and 2 for Si.
where TC is Room temperature in ° C
TK = 25+273=298K
VT Voltage equivalent of temperature
IS Reverse saturation current
Let fLE = 100 Hz
Note: - Some tolerances have been made in order to obtain standard values of components. All capacitors are designed to obtain a fair frequency response curve.
Creating the Library Elements
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.
Place these loaded components in the appropriate position using the Relocate function tool and its option tool(s).
Wires / Nets connection
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.
Mixed Mode 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.
DC Sweep Analysis
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
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.
Mixed Mode Simulator
DC Sweep Analysis
AC Sweep Analysis