Documentation sections

Current Mirror Calculator

POSThttps://rftools.io/api/py/v1/calculate

Slug current-mirror. Calculate current mirror output current, beta error, reference resistor, power consumption, and Widlar mirror resistor for analog IC and bias circuit design

Inputs

Send every one of these in inputs, in the units shown. Most are required: a call without one is refused (400) and costs nothing; one the calculator can do without takes the default shown. An input outside its range is still computed and flagged in the provenance.

Inputs of current-mirror
Name	Unit	Range	Default	Description
refCurrentnumber	mA	at least 0.001	1	Reference Current
rationumber	×	at least 0.001	2	Mirror Ratio (Iout/Iref)
vccnumber	V	at least 0.1	5	Supply Voltage
betanumber	none	at least 1	100	Transistor β (hFE)
vtnumber	mV	10 … 50	26	Thermal voltage kT/q ≈ 26 mV at room temperature
eSeriesnumber	none	0 … 96	0	Rounds every part to the nearest value in the series; the response and the export are the rounded design's.One of: 0 Ideal values · 12 E12 (10%) · 24 E24 (5%) · 48 E48 (2%) · 96 E96 (1%)

Outputs

Returned in values, by key. A value that is unbounded or undefined is null.

Outputs of current-mirror
Field	Unit	Description
outputCurrentheadline	mA	The mirrored current less the β error; with a part series, the current the rounded resistors set
refResistor	kΩ	Sets I_ref from V_CC with V_BE = 0.7 V; with a part series, the nearest value of the series
powerRef	mW	Reference Branch Power
error	%	How far the output current falls short of N × I_ref, in percent (negative: above it): the β error, and with a part series the rounded resistors' error too
widlarResistor	kΩ	Widlar emitter resistor value — only applicable when mirror ratio < 1; with a part series, the nearest value of the series

Calling it

Calculate: one call, the request and response in full, and its errors.
Batch: this calculator with others, or a sweep, in one request.
Solve for a target: the value of one input that brings an output to a target.

curl -X POST https://rftools.io/api/py/v1/calculate \
  -H "X-API-Key: $RFTOOLS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "slug": "current-mirror",
    "inputs": {
      "refCurrent": 1,
      "ratio": 2,
      "vcc": 5,
      "beta": 100,
      "vt": 26,
      "eSeries": 0
    }
  }'

import rftools

client = rftools.Client()  # reads RFTOOLS_API_KEY, or the key `rftools login` saved

result = client.calculate("current-mirror", {
    "refCurrent": 1,
    "ratio": 2,
    "vcc": 5,
    "beta": 100,
    "vt": 26,
    "eSeries": 0,
})
print(result.values)
print(result.warnings, result.provenance.formula_ref)

const response = await fetch("https://rftools.io/api/py/v1/calculate", {
  method: "POST",
  headers: {
    "X-API-Key": process.env.RFTOOLS_API_KEY,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    "slug": "current-mirror",
    "inputs": {
      "refCurrent": 1,
      "ratio": 2,
      "vcc": 5,
      "beta": 100,
      "vt": 26,
      "eSeries": 0
    }
  }),
});
console.log(response.status, await response.json());

package main

import (
	"bytes"
	"fmt"
	"io"
	"net/http"
	"os"
)

func main() {
	body := []byte(`{
	  "slug": "current-mirror",
	  "inputs": {
	    "refCurrent": 1,
	    "ratio": 2,
	    "vcc": 5,
	    "beta": 100,
	    "vt": 26,
	    "eSeries": 0
	  }
	}`)
	req, err := http.NewRequest("POST", "https://rftools.io/api/py/v1/calculate", bytes.NewReader(body))
	if err != nil {
		panic(err)
	}
	req.Header.Set("X-API-Key", os.Getenv("RFTOOLS_API_KEY"))
	req.Header.Set("Content-Type", "application/json")
	res, err := http.DefaultClient.Do(req)
	if err != nil {
		panic(err)
	}
	defer res.Body.Close()
	out, _ := io.ReadAll(res.Body)
	fmt.Println(res.Status, string(out))
}

application/json

{
  "slug": "current-mirror",
  "values": {
    "outputCurrent": 1.9607843137254901,
    "refResistor": 4.3,
    "powerRef": 5,
    "error": 1.9607843137254943,
    "widlarResistor": 0
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:current-mirror",
    "version": "api@3f2c1a9b04de",
    "formulaRef": null,
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "refCurrent": {
          "min": 0.001,
          "max": null,
          "unit": "mA"
        },
        "ratio": {
          "min": 0.001,
          "max": null,
          "unit": "×"
        },
        "vcc": {
          "min": 0.1,
          "max": null,
          "unit": "V"
        },
        "beta": {
          "min": 1,
          "max": null,
          "unit": ""
        },
        "vt": {
          "min": 10,
          "max": 50,
          "unit": "mV"
        },
        "eSeries": {
          "min": 0,
          "max": 96,
          "unit": ""
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "refCurrent": 1,
      "ratio": 2,
      "vcc": 5,
      "beta": 100,
      "vt": 26,
      "eSeries": 0
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "refCurrent"
      ],
      "msg": "Input should be a valid number, unable to parse string as a number",
      "input": "abc"
    }
  ]
}

An input sent as text. Nothing was counted.

{
  "detail": "More than 120 requests in 60 seconds from this address.",
  "errorKind": "rate_limited",
  "limit": 120,
  "windowSeconds": 60,
  "upgradeUrl": "https://rftools.io/pricing/"
}

Too many requests in a minute, with Retry-After.

Same inputs on the calculator pageCurrent Mirror Calculator, at these values