Operational Amplifiers Introduction

Theory of Operation

The low-level In-Phase (I) and Quadrature (Q) IF signals from the Mixer are sampled over capacitors C5 and C6 (in the Mixer stage).

The Opamp amplifies the I and Q signals by about 500 times.

(go directly to build notes)

Operational Amplifiers Schematic

(Resistor testpoints (hairpin, top, or left-hand lead), as physically installed on the board, are marked in the schematic with red dots)

(Click for Full Schematic)
Operational Amplifiersschematic

(above schematic has clickable areas that can be used for navigation)

(go directly to build notes)

Operational Amplifiers Bill of Materials

Stage Bill of Materials

(resistor images and color codes courtesy of WIlfried, DL5SWB's R-Color Code program)

CheckCountComponentMarkingCategory
21 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4W
24.99 k 1/4W 1%y-w-w-br-br y-w-w-br-br1/4W
2100 1/6W 5%br-blk-br-gld br-blk-br-gld1/6W
2390 pF 5%391 391Ceramic
14.7 uF 10% 16V X7R RAD475 475Ceramic
2band-specificmisc
1LT6231 dual op-ampLT6231 LT6231SOIC-8

Band Specific Items for 160m Band

CheckDesignationComponentMarkingCategoryOrientationNotesCircuit
R0549.9 ohm 1%yel-wht-wht-gld-brn yel-wht-wht-gld-brn1/4WE-W Operational Amplifiers
R0649.9 ohm 1%yel-wht-wht-gld-brn yel-wht-wht-gld-brn1/4WE-W Operational Amplifiers

Band Specific Items for 80m Band

CheckDesignationComponentMarkingCategoryOrientationNotesCircuit
R0549.9 ohm 1%yel-wht-wht-gld-brn yel-wht-wht-gld-brn1/4WE-W Operational Amplifiers
R0649.9 ohm 1%yel-wht-wht-gld-brn yel-wht-wht-gld-brn1/4WE-W Operational Amplifiers

Band Specific Items for 40m Band

CheckDesignationComponentMarkingCategoryOrientationNotesCircuit
R0549.9 ohm 1%yel-wht-wht-gld-brn yel-wht-wht-gld-brn1/4WE-W Operational Amplifiers
R0649.9 ohm 1%yel-wht-wht-gld-brn yel-wht-wht-gld-brn1/4WE-W Operational Amplifiers

Band Specific Items for 30m Band

CheckDesignationComponentMarkingCategoryOrientationNotesCircuit
R0510 ohm 1/4W 1%br-blk-blk-gld-br br-blk-blk-gld-br1/4WE-W Operational Amplifiers
R0610 ohm 1/4W 1%br-blk-blk-gld-br br-blk-blk-gld-br1/4WE-W Operational Amplifiers

Band Specific Items for 20m Band

CheckDesignationComponentMarkingCategoryOrientationNotesCircuit
R0510 ohm 1/4W 1%br-blk-blk-gld-br br-blk-blk-gld-br1/4WE-W Operational Amplifiers
R0610 ohm 1/4W 1%br-blk-blk-gld-br br-blk-blk-gld-br1/4WE-W Operational Amplifiers

Operational Amplifiers Summary Build Notes

Operational Amplifiers Detailed Build Notes

Bottom of the Board

Operational Amplifiers Bottom View

Install OpAmp

Install the LT6231 operational amplifier to the bottomside of the board.

See hints on installing SMT ICs.

  • Orient U4 on its pads so that the pin 1 corner of the IC matches the small 1" (it also looks like a 0) mark in the copper on the bottom side of the board. In general, pin 1 of an SOIC packaged IC is in the lower left corner of the package when the printing on the package top reads upright, from left to right.
  • Tack-solder one corner pin of U4 and reheat the tacked pin as necessary to line up U4 on its pads properly.
  • Double-check the orientation of U4 and the line up of the IC on its pads with magnification and good lighting. You do NOT want to install U4 oriented incorrectly. If all is well, carefully solder the rest of the leads to their pads.

Install OpAmp photo
CheckDesignationComponentMarkingCategoryOrientationNotes
U4LT6231 dual op-ampLT6231 LT6231SOIC-8

Top of the Board

Operational Amplifiers Top View

Install band-specific components

Install the band-specific capacitors and resistors.

See band-specific chart for values

See hints on orienting and installing resistors.

See hints on identifying and installing ceramic caps.

CheckDesignationComponentMarkingCategoryOrientationNotes
R05band-specific
BandComponentMarking
160m49.9 ohm 1% (1/4W)yel-wht-wht-gld-brn
80m49.9 ohm 1% (1/4W)yel-wht-wht-gld-brn
40m49.9 ohm 1% (1/4W)yel-wht-wht-gld-brn
30m10 ohm 1/4W 1% (1/4W)br-blk-blk-gld-br
20m10 ohm 1/4W 1% (1/4W)br-blk-blk-gld-br
miscE-W
R06band-specific
BandComponentMarking
160m49.9 ohm 1% (1/4W)yel-wht-wht-gld-brn
80m49.9 ohm 1% (1/4W)yel-wht-wht-gld-brn
40m49.9 ohm 1% (1/4W)yel-wht-wht-gld-brn
30m10 ohm 1/4W 1% (1/4W)br-blk-blk-gld-br
20m10 ohm 1/4W 1% (1/4W)br-blk-blk-gld-br
miscE-W

Install remaining components

See hints on orienting and installing resistors.

See hints on identifying and installing ceramic caps.

CheckDesignationComponentMarkingCategoryOrientationNotes
C08390 pF 5%391 391Ceramic 
C09390 pF 5%391 391Ceramic 
C074.7 uF 10% 16V X7R RAD475 475Ceramic 
R074.99 k 1/4W 1%y-w-w-br-br y-w-w-br-br1/4WS-N
R031 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4WW-E
R084.99 k 1/4W 1%y-w-w-br-br y-w-w-br-br1/4WE-W
R041 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4WW-E
R09100 1/6W 5%br-blk-br-gld br-blk-br-gld1/6WE-W
R10100 1/6W 5%br-blk-br-gld br-blk-br-gld1/6WE-W

Operational Amplifiers Testing

Warning

Test Setup

Take appropriate ESD precautions in these tests, since you will be working around the sensitive OpAmp IC

Visual Check

Test Setup

Using very good lighting and magnification, carefully inspect the solder joints to identify bridges, cold joints, or poor contacts.

Pay especial attention to the joints on the OpAmp IC pins. If necessary, touch up the joints with your iron and/or some flux. Wick up any excess.

Current Draw

Test Setup

  • In each test, the ammeter must be placed in series between the positive lead of the power source and the board's positive power-in "+" terminal.
  • In one test there is also a 100 ohm resistor in the series "chain" as well.
  • in the second test, the setup is the same except that the current-liminting resistor is removed

Apply 12 Vdc to the board for this test

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
With 100 ohm current-limiting resistormA< 3026.6_______
Without current limiting resistormA< 3026.9_______

Voltage Tests

Test Setup

Measure the voltages with respect to ground for each of the pins of U4. Tage care to measure at the actual IC pin rather than the pad, so as to ensure you are measuring the PIN voltage

expected voltages are indicated in the table below:

  • 5 V (range of 4.5 - 5.4)
  • 2.5 V (approx 50% of the 5V rail value)
Voltage Tests

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
Pin 1Vdc2.52.46_______
Pin 2Vdc2.52.46_______
Pin 3Vdc2.52.46_______
Pin 4Vdc00_______
Pin 5Vdc2.52.46_______
Pin 6Vdc2.52.46_______
Pin 7Vdc2.52.46_______
Pin 8Vdc54.94_______

Functional Test

Test Setup

You will test the DC gain of each of the op-amps by connecting resistors RB from each op-amp inverting input to circuit ground. Introducing the "bridging" resistor RB will result in a test current equal to 2.5 / Rt which will be balanced by the current fed back from each op-amp's output through each feedback resistor, RF (i.e., R7 or R8). Each op-amp output will increase in voltage by 2.5 * RF/ RB from the nominal DC level of 2.5 volts.

The value of the "bridging" resistor (RB) is 10 kΩ:

  • Test the First OpAmp

    Power up the circuit and measure the voltage at pin 1 of the op-amp (hairpin of R8). It should be ~2.5 Vdc
  • Power off and use clip leads to connect RB between the hairpin of R6 and circuit ground. (This provides an input resistance(Ri) of 10 kΩ
  • Power up and measure the output voltage at the hairpin of the feedback resistor R8. You should get:, with RB=10 kΩ and R8=4.99 kΩ: ~3.75 Vdc.
  • Remove RB and the output voltage at R8 should go back to ~2.5 Vdc.
  • Test the Second OpAmp

    Power up the circuit and measure the voltage at pin 7 of the op-amp (hairpin of R7). It should be ~2.5 Vdc
  • Power off and use clip leads to connect RB between the hairpin of R5 and circuit ground. (This provides an input resistance(Ri) of 10 kΩ
  • Power up and measure the output voltage at the hairpin of the feedback resistor R7. You should get, with RB=10 kΩ and R7=4.99 kΩ: ~3.75 Vdc.
  • Remove RB and the output voltage at R7 should go back to ~2.5 Vdc.

The diagram below shows the test points. The yellow dots show the output voltage measurement points. The white points show the bridging resistor connection points (connect the bridging resistor RB between the ground and a white point). To measure the voltage at yellow point "A", use white point "A" for the bridge; same with points "B".

An Excel spreadsheet with a calculator for this test is available for you to plug in your bridging resistor ohms (Rt) and your pin 1 or pin 7 normal voltages (Ebias) and predict the expected voltage when bridged (Eout).

<a name=Functional Test"

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
R7 (yellow "A")- unbridgedVdc2.52.48_______
R8 (yellow "B") - unbridgedVdc2.52.46_______
R7 (yellow "A") bridging white "A" to groundVdc3.753.72_______
R8 (yellow "B") bridging white "B" to groundVdc3.753.69_______
link to BPF Stage link to Power Supply Stage link to Mixer Stage link to External Connections Stage